PYTHON-5055 - Convert test_client.py to unittest

[NoahStapp]

Main points of note:

1. All stateful unittest methods have been converted to pytest fixtures.
2. Every async test and fixture runs on the same loop that exists for the entire test session. This is separate from the pytest caching scope for fixtures, which is controlled by scope instead of loop_scope.
3. ClientContext has been kept as a metadata store for our tests served by a session-scoped fixture.
4. unit and integration pytest marks have been added. Going forward, all test classes should be marked with the appropriate one.

[blink1073]

evergreen retry

[sleepyStick]

I think the Python changes look good to me. (I found things that I needed to change on my PR / test conversions as a result of reviewing this so that was helpful haha)