Skip unavailable requested loop factories

[tjkuson]

If a user requests a loop factory that is not available, skip that test variant instead of raising an error.

Resolves #1101 (comment)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.11%. Comparing base (bad3415) to head (8dece7f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1410      +/-   ##
==========================================
- Coverage   95.13%   95.11%   -0.03%     
==========================================
  Files           2        2              
  Lines         473      471       -2     
  Branches       57       56       -1     
==========================================
- Hits          450      448       -2     
  Misses         17       17              
  Partials        6        6              

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