Expand tests for fill transformations

[paulromano]

This PR expands the test for fill transformations to handle different cases for how it can be specified.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.66%. Comparing base (0b11ae4) to head (64c464b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   81.90%   84.66%   +2.76%     
==========================================
  Files           3        3              
  Lines         724      724              
  Branches      190      190              
==========================================
+ Hits          593      613      +20     
+ Misses         73       57      -16     
+ Partials       58       54       -4     

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