For the Completer unit tests:
Although `change_completed_part` looks like a public method, it was
only used internally. Test the externally-used method
`selection_changed` instead.
Based on code review:
- Use qtbot.waitSignal to test a signal firing
- Use pytest.mark.xfail for an expected test failure
- Ensure there are 2 newlines between module-level functions
Based on code review:
- import modules, not classes
- use methods, not lambdas for the mock command prompt class
- use None rather than Mock for DUMB_SORT
- autouse two fixtures and remove them from test signatures