Add a unit test for the CO2 corrector

[gerritholl]

Add a unit test for satpy.modifiers.atmosphere.CO2Corrector.

Also move other atmospheric modifier unit tests to their own test module.

• Tests added

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.35%. Comparing base (6c571f2) to head (bf75088).
⚠️ Report is 77 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3371   +/-   ##
=======================================
  Coverage   96.35%   96.35%           
=======================================
  Files         466      467    +1     
  Lines       59083    59106   +23     
=======================================
+ Hits        56931    56954   +23     
  Misses       2152     2152           

Flag	Coverage Δ
behaviourtests	3.58% <0.00%> (-0.01%)	⬇️
unittests	96.44% <100.00%> (+<0.01%)	⬆️

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[gerritholl]

CodeScene is complaining about code that I moved but did not otherwise touch. An attempt to suppress the finding still fails with "Error occurred while suppressing! Please try again." I don't know if the suppression has ever worked (for me it never has).

[djhoese]

Maybe I'm misunderstanding, but we don't want to suppress what CodeScene is finding since it is an actual problem. We just won't let it stop this PR from being merged as it isn't a new issue you are adding.