Add pytest-style rules for ruff

[brisvag]

References and relevant issues

Tracking issue: #5589 Followup on #6840 (though independent).

Description

Add the PT rule to check use of fixtures and other pytest things. It fixed some formal stuff, but also caught quite a few mistakes.

The two main changes are:

• fixtures naming conventions (leading _ is for those that don't return an object)
• all the pytest.raises(ValueError) needed an appropriate match= parameter to be less vague

[brisvag]

These failures are confusing... @Czaki any ideas? ^^'

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.12536% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 92.40%. Comparing base (a491e60) to head (8afecec). Report is 1 commits behind head on main.

Files	Patch %	Lines
...layers/points/_tests/test_points_mouse_bindings.py	66.66%	1 Missing :warning:
...layers/shapes/_tests/test_shapes_mouse_bindings.py	50.00%	1 Missing :warning:
napari/utils/_testsupport.py	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6842 +/- ## ========================================== - Coverage 92.45% 92.40% -0.05% ========================================== Files 614 614 Lines 54856 54862 +6 ========================================== - Hits 50715 50695 -20 - Misses 4141 4167 +26 ```

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