Convert CF Writer tests to pytest

[ghiggi]

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

Convert CFWriter test units from unitest to pytest

[codecov]

Codecov Report

Merging #2523 (a7f28e0) into main (bcd6581) will decrease coverage by 0.09%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##             main    #2523      +/-   ##
==========================================
- Coverage   94.96%   94.88%   -0.09%     
==========================================
  Files         347      347              
  Lines       50921    50931      +10     
==========================================
- Hits        48358    48326      -32     
- Misses       2563     2605      +42     

Flag	Coverage Δ
behaviourtests	4.31% <0.00%> (-0.01%)	⬇️
unittests	95.48% <86.66%> (-0.09%)	⬇️

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Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_cf.py	95.53% <86.66%> (-4.06%)	⬇️

... and 7 files with indirect coverage changes

[ghiggi]

mocks not needed I guess ...

[coveralls]

Pull Request Test Coverage Report for Build 5392538598

• 130 of 150 (86.67%) changed or added relevant lines in 1 file are covered.
• 11 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.06%) to 95.46%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/tests/writer_tests/test_cf.py	130	150	86.67%

Files with Coverage Reduction	New Missed Lines	%
satpy/writers/cf_writer.py	1	94.9%
satpy/tests/writer_tests/test_cf.py	10	95.53%

Totals
Change from base Build 5389825507:	-0.06%
Covered Lines:	48450
Relevant Lines:	50754

---

💛 - Coveralls

[ghiggi]

@djhoese I think is ready

[djhoese]

Looks good. For the record, you could have used setup_method instead of converting the fake datasets to fixtures, but I'm guessing you knew that. Thanks for cleaning this up. Let's merge it...