Add image cache testing for Mac and Windows

[akaszynski]

Add in image cache testing for MacOS and Windows based on the comment in #2441.

Changes:

• Removed cache for test_plot as we're using full_screen=True here and this does not work with unit testing within MacOS. The full_screen parameter actually changes the window since, while on Linux it does not.
• Several tests fail on Windows when using (and not using) OpenGL compiled with OSMesa. Rather than having a separate test suite with these images, this PR just skips them.

[codecov]

Codecov Report

Merging #2716 (b0a6a86) into main (975645d) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2716      +/-   ##
==========================================
- Coverage   93.82%   93.76%   -0.07%     
==========================================
  Files          75       75              
  Lines       16148    16148              
==========================================
- Hits        15151    15141      -10     
- Misses        997     1007      +10     

[adeak]

Just so I understand correctly, this means that we don't need a separate image cache for each OS after all, at the cost of skipping this small handful of flaky tests?

[akaszynski]

Just so I understand correctly, this means that we don't need a separate image cache for each OS after all, at the cost of skipping this small handful of flaky tests?

Exactly. MacOS only needed one change, Windows would have just needed two skips but for OSMesa. 90% testing is better than no testing.

[adeak]

Looks good to me as-is, thanks. One very small suggestion that's not critical at all.

[adeak]

Thanks!