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, 103 deletions

diff --git a/tests/cogs/sync/test_roles.py b/tests/cogs/sync/test_roles.py
deleted file mode 100644
index c561ba447..000000000
--- a/tests/cogs/sync/test_roles.py
+++ /dev/null
@@ -1,103 +0,0 @@
-from bot.cogs.sync.syncers import Role, get_roles_for_sync
-
-
-def test_get_roles_for_sync_empty_return_for_equal_roles():
-    api_roles = {Role(id=41, name='name', colour=33, permissions=0x8, position=1)}
-    guild_roles = {Role(id=41, name='name', colour=33, permissions=0x8, position=1)}
-
-    assert get_roles_for_sync(guild_roles, api_roles) == (set(), set(), set())
-
-
-def test_get_roles_for_sync_returns_roles_to_update_with_non_id_diff():
-    api_roles = {Role(id=41, name='old name', colour=35, permissions=0x8, position=1)}
-    guild_roles = {Role(id=41, name='new name', colour=33, permissions=0x8, position=2)}
-
-    assert get_roles_for_sync(guild_roles, api_roles) == (
-        set(),
-        guild_roles,
-        set(),
-    )
-
-
-def test_get_roles_only_returns_roles_that_require_update():
-    api_roles = {
-        Role(id=41, name='old name', colour=33, permissions=0x8, position=1),
-        Role(id=53, name='other role', colour=55, permissions=0, position=3)
-    }
-    guild_roles = {
-        Role(id=41, name='new name', colour=35, permissions=0x8, position=2),
-        Role(id=53, name='other role', colour=55, permissions=0, position=3)
-    }
-
-    assert get_roles_for_sync(guild_roles, api_roles) == (
-        set(),
-        {Role(id=41, name='new name', colour=35, permissions=0x8, position=2)},
-        set(),
-    )
-
-
-def test_get_roles_returns_new_roles_in_first_tuple_element():
-    api_roles = {
-        Role(id=41, name='name', colour=35, permissions=0x8, position=1),
-    }
-    guild_roles = {
-        Role(id=41, name='name', colour=35, permissions=0x8, position=1),
-        Role(id=53, name='other role', colour=55, permissions=0, position=2)
-    }
-
-    assert get_roles_for_sync(guild_roles, api_roles) == (
-        {Role(id=53, name='other role', colour=55, permissions=0, position=2)},
-        set(),
-        set(),
-    )
-
-
-def test_get_roles_returns_roles_to_update_and_new_roles():
-    api_roles = {
-        Role(id=41, name='old name', colour=35, permissions=0x8, position=1),
-    }
-    guild_roles = {
-        Role(id=41, name='new name', colour=40, permissions=0x16, position=2),
-        Role(id=53, name='other role', colour=55, permissions=0, position=3)
-    }
-
-    assert get_roles_for_sync(guild_roles, api_roles) == (
-        {Role(id=53, name='other role', colour=55, permissions=0, position=3)},
-        {Role(id=41, name='new name', colour=40, permissions=0x16, position=2)},
-        set(),
-    )
-
-
-def test_get_roles_returns_roles_to_delete():
-    api_roles = {
-        Role(id=41, name='name', colour=35, permissions=0x8, position=1),
-        Role(id=61, name='to delete', colour=99, permissions=0x9, position=2),
-    }
-    guild_roles = {
-        Role(id=41, name='name', colour=35, permissions=0x8, position=1),
-    }
-
-    assert get_roles_for_sync(guild_roles, api_roles) == (
-        set(),
-        set(),
-        {Role(id=61, name='to delete', colour=99, permissions=0x9, position=2)},
-    )
-
-
-def test_get_roles_returns_roles_to_delete_update_and_new_roles():
-    api_roles = {
-        Role(id=41, name='not changed', colour=35, permissions=0x8, position=1),
-        Role(id=61, name='to delete', colour=99, permissions=0x9, position=2),
-        Role(id=71, name='to update', colour=99, permissions=0x9, position=3),
-    }
-    guild_roles = {
-        Role(id=41, name='not changed', colour=35, permissions=0x8, position=1),
-        Role(id=81, name='to create', colour=99, permissions=0x9, position=4),
-        Role(id=71, name='updated', colour=101, permissions=0x5, position=3),
-    }
-
-    assert get_roles_for_sync(guild_roles, api_roles) == (
-        {Role(id=81, name='to create', colour=99, permissions=0x9, position=4)},
-        {Role(id=71, name='updated', colour=101, permissions=0x5, position=3)},
-        {Role(id=61, name='to delete', colour=99, permissions=0x9, position=2)},
-    )