Migrate pytest to unittest

After a discussion in the core developers channel, we have decided to
migrate from `pytest` to `unittest` as the testing framework. This
commit sets up the repository to use `unittest` and migrates the
first couple of tests files to the new framework.

What I have done to migrate to `unitest`:

- Removed all `pytest` test files, since they are incompatible.

- Removed `pytest`-related dependencies from the Pipfile.

- Added `coverage.py` to the Pipfile dev-packages and relocked.

- Added convenience scripts to Pipfile for running the test suite.

- Adjust to `azure-pipelines.yml` to use `coverage.py` and `unittest`.

- Migrated four test files from `pytest` to `unittest` format.

In addition, I've added five helper Mock subclasses in `helpers.py`
and created a `TestCase` subclass in `base.py` to add an assertion
that asserts that no log records were logged within the context of
the context manager. Obviously, these new utility functions and
classes are fully tested in their respective `test_` files.

Finally, I've started with an introductory guide for writing tests
for our bot in `README.md`.

1 files changed, 0 insertions, 211 deletions

diff --git a/tests/cogs/test_information.py b/tests/cogs/test_information.py
deleted file mode 100644
index 184bd2595..000000000
--- a/tests/cogs/test_information.py
+++ /dev/null
@@ -1,211 +0,0 @@
-import asyncio
-import logging
-import textwrap
-from datetime import datetime
-from unittest.mock import MagicMock, patch
-
-import pytest
-from discord import (
-    CategoryChannel,
-    Colour,
-    Permissions,
-    Role,
-    TextChannel,
-    VoiceChannel,
-)
-
-from bot.cogs import information
-from bot.constants import Emojis
-from bot.decorators import InChannelCheckFailure
-from tests.helpers import AsyncMock
-
-
[email protected]()
-def cog(simple_bot):
-    return information.Information(simple_bot)
-
-
-def role(name: str, id_: int):
-    r = MagicMock()
-    r.name = name
-    r.id = id_
-    r.mention = f'&{name}'
-    return r
-
-
-def member(status: str):
-    m = MagicMock()
-    m.status = status
-    return m
-
-
[email protected]()
-def ctx(moderator_role, simple_ctx):
-    simple_ctx.author.roles = [moderator_role]
-    simple_ctx.guild.created_at = datetime(2001, 1, 1)
-    simple_ctx.send = AsyncMock()
-    return simple_ctx
-
-
-def test_roles_info_command(cog, ctx):
-    everyone_role = MagicMock()
-    everyone_role.name = '@everyone'  # should be excluded in the output
-    ctx.author.roles.append(everyone_role)
-    ctx.guild.roles = ctx.author.roles
-
-    cog.roles_info.can_run = AsyncMock()
-    cog.roles_info.can_run.return_value = True
-
-    coroutine = cog.roles_info.callback(cog, ctx)
-
-    assert asyncio.run(coroutine) is None  # no rval
-    ctx.send.assert_called_once()
-    _, kwargs = ctx.send.call_args
-    embed = kwargs.pop('embed')
-    assert embed.title == "Role information"
-    assert embed.colour == Colour.blurple()
-    assert embed.description == f"`{ctx.guild.roles[0].id}` - {ctx.guild.roles[0].mention}\n"
-    assert embed.footer.text == "Total roles: 1"
-
-
-def test_role_info_command(cog, ctx):
-    dummy_role = MagicMock(spec=Role)
-    dummy_role.name = "Dummy"
-    dummy_role.colour = Colour.blurple()
-    dummy_role.id = 112233445566778899
-    dummy_role.position = 10
-    dummy_role.permissions = Permissions(0)
-    dummy_role.members = [ctx.author]
-
-    admin_role = MagicMock(spec=Role)
-    admin_role.name = "Admin"
-    admin_role.colour = Colour.red()
-    admin_role.id = 998877665544332211
-    admin_role.position = 3
-    admin_role.permissions = Permissions(0)
-    admin_role.members = [ctx.author]
-
-    ctx.guild.roles = [dummy_role, admin_role]
-
-    cog.role_info.can_run = AsyncMock()
-    cog.role_info.can_run.return_value = True
-
-    coroutine = cog.role_info.callback(cog, ctx, dummy_role, admin_role)
-
-    assert asyncio.run(coroutine) is None
-
-    assert ctx.send.call_count == 2
-
-    (_, dummy_kwargs), (_, admin_kwargs) = ctx.send.call_args_list
-
-    dummy_embed = dummy_kwargs["embed"]
-    admin_embed = admin_kwargs["embed"]
-
-    assert dummy_embed.title == "Dummy info"
-    assert dummy_embed.colour == Colour.blurple()
-
-    assert dummy_embed.fields[0].value == str(dummy_role.id)
-    assert dummy_embed.fields[1].value == f"#{dummy_role.colour.value:0>6x}"
-    assert dummy_embed.fields[2].value == "0.63 0.48 218"
-    assert dummy_embed.fields[3].value == "1"
-    assert dummy_embed.fields[4].value == "10"
-    assert dummy_embed.fields[5].value == "0"
-
-    assert admin_embed.title == "Admin info"
-    assert admin_embed.colour == Colour.red()
-
-# There is no argument passed in here that we can use to test,
-# so the return value would change constantly.
-@patch('bot.cogs.information.time_since')
-def test_server_info_command(time_since_patch, cog, ctx, moderator_role):
-    time_since_patch.return_value = '2 days ago'
-
-    ctx.guild.created_at = datetime(2001, 1, 1)
-    ctx.guild.features = ('lemons', 'apples')
-    ctx.guild.region = 'The Moon'
-    ctx.guild.roles = [moderator_role]
-    ctx.guild.channels = [
-        TextChannel(
-            state={},
-            guild=ctx.guild,
-            data={'id': 42, 'name': 'lemons-offering', 'position': 22, 'type': 'text'}
-        ),
-        CategoryChannel(
-            state={},
-            guild=ctx.guild,
-            data={'id': 5125, 'name': 'the-lemon-collection', 'position': 22, 'type': 'category'}
-        ),
-        VoiceChannel(
-            state={},
-            guild=ctx.guild,
-            data={'id': 15290, 'name': 'listen-to-lemon', 'position': 22, 'type': 'voice'}
-        )
-    ]
-    ctx.guild.members = [
-        member('online'), member('online'),
-        member('idle'),
-        member('dnd'), member('dnd'), member('dnd'), member('dnd'),
-        member('offline'), member('offline'), member('offline')
-    ]
-    ctx.guild.member_count = 1_234
-    ctx.guild.icon_url = 'a-lemon.png'
-
-    coroutine = cog.server_info.callback(cog, ctx)
-    assert asyncio.run(coroutine) is None  # no rval
-
-    time_since_patch.assert_called_once_with(ctx.guild.created_at, precision='days')
-    _, kwargs = ctx.send.call_args
-    embed = kwargs.pop('embed')
-    assert embed.colour == Colour.blurple()
-    assert embed.description == textwrap.dedent(f"""
-        **Server information**
-        Created: {time_since_patch.return_value}
-        Voice region: {ctx.guild.region}
-        Features: {', '.join(ctx.guild.features)}
-
-        **Counts**
-        Members: {ctx.guild.member_count:,}
-        Roles: {len(ctx.guild.roles)}
-        Text: 1
-        Voice: 1
-        Channel categories: 1
-
-        **Members**
-        {Emojis.status_online} 2
-        {Emojis.status_idle} 1
-        {Emojis.status_dnd} 4
-        {Emojis.status_offline} 3
-        """)
-    assert embed.thumbnail.url == 'a-lemon.png'
-
-
-def test_user_info_on_other_users_from_non_moderator(ctx, cog):
-    ctx.author = MagicMock()
-    ctx.author.__eq__.return_value = False
-    ctx.author.roles = []
-    coroutine = cog.user_info.callback(cog, ctx, user='scragly')  # skip checks, pass args
-
-    assert asyncio.run(coroutine) is None  # no rval
-    ctx.send.assert_called_once_with(
-        "You may not use this command on users other than yourself."
-    )
-
-
-def test_user_info_in_wrong_channel_from_non_moderator(ctx, cog):
-    ctx.author = MagicMock()
-    ctx.author.__eq__.return_value = False
-    ctx.author.roles = []
-
-    coroutine = cog.user_info.callback(cog, ctx)
-    message = 'Sorry, but you may only use this command within <#267659945086812160>.'
-    with pytest.raises(InChannelCheckFailure, match=message):
-        assert asyncio.run(coroutine) is None  # no rval
-
-
-def test_setup(simple_bot, caplog):
-    information.setup(simple_bot)
-    simple_bot.add_cog.assert_called_once()
-    [record] = caplog.records
-
-    assert record.message == "Cog loaded: Information"
-    assert record.levelno == logging.INFO