Convert TestYAMLFiles to pytest

[djhoese]

This should hopefully fix some leaking monkeypatch modifications. My theory based on the failures seen in the unstable/test environment is that this monkeypatch'd import was being used in other tests besides the one it was implemented in (tracebacks include the real_import name). It may not be the actual solution to the failures in the experimental environment, but it is a start.

CC @BENR0

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2404 (431032c) into main (16d3184) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2404      +/-   ##
==========================================
- Coverage   94.73%   94.73%   -0.01%     
==========================================
  Files         329      329              
  Lines       48739    48736       -3     
==========================================
- Hits        46171    46168       -3     
  Misses       2568     2568              

Flag	Coverage Δ
behaviourtests	4.41% <0.00%> (+<0.01%)	⬆️
unittests	95.36% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
satpy/tests/enhancement_tests/test_ahi.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_fy4_base.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_ghi_l1.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_msi_safe.py	100.00% <100.00%> (ø)
satpy/tests/reader_tests/test_msu_gsa_l1b.py	100.00% <100.00%> (ø)
...tpy/tests/reader_tests/test_oceancolorcci_l3_nc.py	100.00% <100.00%> (ø)
satpy/tests/test_readers.py	98.89% <100.00%> (-0.01%)	⬇️

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[coveralls]

Coverage: 95.319% (-0.0003%) from 95.319% when pulling 431032c on djhoese:tests-leaking-imports into 16d3184 on pytroll:main.

[djhoese]

I thought I mentioned it here, but I guess I only commented on slack. The CI failure is caused by an incompatibility between holoviews and dev numpy. See:

numpy/numpy#23307

and:

holoviz/holoviews#5635

[mraspaud]

LGTM, thanks for the modernisation!