Fix positive attributes in custom CMOR variables

[schlunma]

Description

This PR fixes positive attributes in custom CMOR variables:

• For ISCCP cloud are fractions, remove it (it does not make sense to have it here)
• For the cloud radiative effects (CREs), change it from up to down
• For TOA Net downward Shortwave Radiation, change it from up to down

This PR also adds missing tests for these derived variables to improve our overall test coverage.

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Checklist

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• 🛠 Unit tests have been added
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.25%. Comparing base (3a0b49f) to head (e3a33e7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2380      +/-   ##
==========================================
+ Coverage   94.10%   94.25%   +0.15%     
==========================================
  Files         246      246              
  Lines       13477    13483       +6     
==========================================
+ Hits        12682    12708      +26     
+ Misses        795      775      -20     

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[valeriupredoi]

cheers, Manu! I think you've fixed a bunch of historical bugs I put in many years ago too, if I'm not mistaken 🍺