Add extra garbage collection for some viewer tests

[andy-sweet]

Description

This adds a special call to gc.collect to one of the viewer tests, so that any Viewer and QtViewer instances should not be present in future tests. That's particularly important for tests that call make_napari_viewer, which asserts that there are no QtViewer instances.

The hope is that this fixes the failing py310-windows-pyside2 tests on main. I was able to reproduce that failure and this fix did fix it.

Type of change

• Bug-fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How has this been tested?

• all tests pass with my change

Final checklist:

• My PR is the minimum possible work for the desired functionality
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• If I included new strings, I have used trans. to make them localizable.
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[github-actions]

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[codecov]

Codecov Report

Merging #5108 (4b73273) into main (6cf965e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5108      +/-   ##
==========================================
- Coverage   87.62%   87.61%   -0.01%     
==========================================
  Files         587      587              
  Lines       50049    50051       +2     
==========================================
  Hits        43854    43854              
- Misses       6195     6197       +2     

Impacted Files	Coverage Δ
napari/_tests/test_view_layers.py	98.31% <100.00%> (+0.02%)	⬆️
...layers/image/experimental/_chunked_image_loader.py	88.00% <0.00%> (-8.00%)	⬇️
napari/layers/image/_image_slice.py	97.61% <0.00%> (-2.39%)	⬇️
napari/layers/image/image.py	95.59% <0.00%> (-0.25%)	⬇️
napari/utils/theme.py	89.79% <0.00%> (+0.68%)	⬆️
napari/utils/info.py	83.33% <0.00%> (+1.11%)	⬆️

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