attr.gi code coverage

[mtorpey]

These PR fixes the four untested sections of code from attr.gi, some by adding tests, and some by amending the code.

The four commits are logically separate, so it might be better not to squash.

Code coverage of attr.gi should now be 100%.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.41%. Comparing base (2de850a) to head (97657a0).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #919      +/-   ##
==========================================
+ Coverage   97.37%   97.41%   +0.03%     
==========================================
  Files          50       50              
  Lines       21179    21175       -4     
  Branches      639      639              
==========================================
+ Hits        20623    20627       +4     
+ Misses        491      483       -8     
  Partials       65       65              

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[james-d-mitchell]

Nice, thanks @mtorpey