Migrate tests from `unittest` to `pytest`

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