chore: refactor some of the loader tests in mdatagen to be more unity, less integration-like

[dmathieu]

Several of the loader_test tests in mdatagen are very much integration-like, and setup a lot of things, when they really should be unit tests.

Every remaining test in loader_test should be dependent on the full mdatagen metadata.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.20%. Comparing base (a58f96f) to head (3cc0367).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15025      +/-   ##
==========================================
- Coverage   91.22%   91.20%   -0.02%     
==========================================
  Files         697      697              
  Lines       44627    44627              
==========================================
- Hits        40710    40704       -6     
- Misses       2773     2776       +3     
- Partials     1144     1147       +3     

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