Migrate tests from `unittest` to `pytest`

[snejus]

Following our discussion under https://github.com/beetbox/beets/issues/5229, I am adding this issue to track the progress.

Proposed solution

Make our tests use pytest instead of unittest.

Objective

This updates our testing codebase to be based on the currently accepted best practice - pytest.

Goals

Non-goals

Anti-goals

[bal-e]

I'd like to take on this, though most of it will have to wait until #5362, #5363, and #5229 are resolved. Once #5362 is complete, I'm happy to start refactoring some of the smaller test modules in small PR batches.