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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• [x] 🧪 The new functionality is relevant and scientifically sound
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• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🧪 and 🛠 Documentation is available
• [x] 🛠 Unit tests have been added
• [x] 🛠 Changes are backward compatible
• [x] 🛠 Any changed dependencies have been added or removed correctly
• [x] 🛠 The list of authors is up to date
• [x] 🛠 All checks below this pull request were successful

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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