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, 66 deletions

diff --git a/tests/utils/test_checks.py b/tests/utils/test_checks.py
deleted file mode 100644
index 7121acebd..000000000
--- a/tests/utils/test_checks.py
+++ /dev/null
@@ -1,66 +0,0 @@
-from unittest.mock import MagicMock
-
-import pytest
-
-from bot.utils import checks
-
-
[email protected]()
-def context():
-    return MagicMock()
-
-
-def test_with_role_check_without_guild(context):
-    context.guild = None
-
-    assert not checks.with_role_check(context)
-
-
-def test_with_role_check_with_guild_without_required_role(context):
-    context.guild = True
-    context.author.roles = []
-
-    assert not checks.with_role_check(context)
-
-
-def test_with_role_check_with_guild_with_required_role(context):
-    context.guild = True
-    role = MagicMock()
-    role.id = 42
-    context.author.roles = (role,)
-
-    assert checks.with_role_check(context, role.id)
-
-
-def test_without_role_check_without_guild(context):
-    context.guild = None
-
-    assert not checks.without_role_check(context)
-
-
-def test_without_role_check_with_unwanted_role(context):
-    context.guild = True
-    role = MagicMock()
-    role.id = 42
-    context.author.roles = (role,)
-
-    assert not checks.without_role_check(context, role.id)
-
-
-def test_without_role_check_without_unwanted_role(context):
-    context.guild = True
-    role = MagicMock()
-    role.id = 42
-    context.author.roles = (role,)
-
-    assert checks.without_role_check(context, role.id + 10)
-
-
-def test_in_channel_check_for_correct_channel(context):
-    context.channel.id = 42
-    assert checks.in_channel_check(context, context.channel.id)
-
-
-def test_in_channel_check_for_incorrect_channel(context):
-    context.channel.id = 42
-    assert not checks.in_channel_check(context, context.channel.id + 10)