TST: migrate image comparison tests from nose to pytest-mpl (⏰ wait on #4122)

[neutrinoceros]

PR Summary

Partially addresses #4102

This is far from done, but I need early iterations with CI to see what problems I encounter.
Right now I'm only aiming at image tests being skipped on nose

approach:
the main difficulty is that our yield base image tests are built via loops, while pytest-mpl relies on return values of test functions. When pytest.mark.parametrize is appropriate, this can be a one-to-one translation, but otherwise I rely on generated code to reproduce existing test (by which I mean that I write many alike plain test functions, just not purely by hand).

TODO:

• stop using image answer tests (replace them with pytest-mpl)
  • GenericImageTest
  • VRImageComparisonTest
  • PhasePlotAttributeTest
  • PixelizedParticleProjectionValuesTest
  • PixelizedProjectionValuesTest
  • PlotWindowAttributeTest
• create image baseline folder (should probably be a submodule, not sure if a new one is warranted, or if we should just reuse answer-store)
• add image image comparison-only test job
• cleanup tests/report_failed_answers.py
• cleanup uses of filename=... in mpl_image_compar markers
• consider refactoring again with pytest.mark.parametrize

opened questions:

• How can we handle "big data" image tests, and more generally image tests that use real (large) data ? Currently these tests are run on Jenkins + nose

[neutrinoceros]

I was not originally planning to fully replace the "answer tests" job on GHA but I see that after removing all image tests from it, it now runs only a very small set of tests and takes about 20s to run (and 10min to build), so I think I should just clear that out completely and replace the job with a new one 100% focused on image-comparison tests.

[neutrinoceros]

I think I should split this in two parts and separate tests that require real data (running on Jenkins) from the rest (running on GH)

[neutrinoceros]

This is probably beyond saving at this point. In any case, #4122 has most of this work in a mergeable state, so I think I should just close this one.

[matthewturk]

Can you describe the difference with #4122?

[neutrinoceros]

Mostly, I was initially going for a wider target, and wanted to migrate all image tests at once, including the ones that require large datasets that are only run on Jenkins. In #4122 I just focused on GHA