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, 54 deletions

diff --git a/tests/cogs/test_security.py b/tests/cogs/test_security.py
deleted file mode 100644
index 1efb460fe..000000000
--- a/tests/cogs/test_security.py
+++ /dev/null
@@ -1,54 +0,0 @@
-import logging
-from unittest.mock import MagicMock
-
-import pytest
-from discord.ext.commands import NoPrivateMessage
-
-from bot.cogs import security
-
-
[email protected]()
-def cog():
-    bot = MagicMock()
-    return security.Security(bot)
-
-
[email protected]()
-def context():
-    return MagicMock()
-
-
-def test_check_additions(cog):
-    cog.bot.check.assert_any_call(cog.check_on_guild)
-    cog.bot.check.assert_any_call(cog.check_not_bot)
-
-
-def test_check_not_bot_for_humans(cog, context):
-    context.author.bot = False
-    assert cog.check_not_bot(context)
-
-
-def test_check_not_bot_for_robots(cog, context):
-    context.author.bot = True
-    assert not cog.check_not_bot(context)
-
-
-def test_check_on_guild_outside_of_guild(cog, context):
-    context.guild = None
-
-    with pytest.raises(NoPrivateMessage, match="This command cannot be used in private messages."):
-        cog.check_on_guild(context)
-
-
-def test_check_on_guild_on_guild(cog, context):
-    context.guild = "lemon's lemonade stand"
-    assert cog.check_on_guild(context)
-
-
-def test_security_cog_load(caplog):
-    bot = MagicMock()
-    security.setup(bot)
-    bot.add_cog.assert_called_once()
-    [record] = caplog.records
-    assert record.message == "Cog loaded: Security"
-    assert record.levelno == logging.INFO