test(isUUID): add tests for nil/max/all options

[WikiRik]

Adds some tests to isUUID to cover the nil/max/all options, and a test for an invalid option. This makes codecov report a 100% project coverage again.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (791ef16) to head (8ab988d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #2549      +/-   ##
===========================================
+ Coverage   99.96%   100.00%   +0.03%     
===========================================
  Files         114       114              
  Lines        2535      2535              
  Branches      641       641              
===========================================
+ Hits         2534      2535       +1     
+ Partials        1         0       -1     

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