Ensure CMOR tables are loaded and match configuration when running tests

[bouweandela]

Description

Ensure that the global variable esmvalcore.cmor.table.CMOR_TABLES has been populated and matches the configuration when running tests.

Closes #3069

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.16%. Comparing base (4ea959b) to head (d54abc7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3086   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files         270      270           
  Lines       15825    15825           
=======================================
  Hits        15218    15218           
  Misses        607      607           

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[valeriupredoi]

🐒 🩹 well done, thanks, bud 🍺