test(assert): improve test coverage

[mbhrznr]

working towards #3713.
pushes code coverage of the assert sub-module to 100%.

primarily some smaller missing tests, such as the often optional msg arg and/or testing for invalid input.
slightly tweaks some smaller implementation details, where the code was unreachable during testing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.47%. Comparing base (5e99c21) to head (606146c).

❗ Current head 606146c differs from pull request most recent head a847a85. Consider uploading reports for the commit a847a85 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4679      +/-   ##
==========================================
+ Coverage   91.36%   91.47%   +0.11%     
==========================================
  Files         477      477              
  Lines       37332    37330       -2     
  Branches     5323     5328       +5     
==========================================
+ Hits        34107    34149      +42     
+ Misses       3164     3125      -39     
+ Partials       61       56       -5     

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[iuioiua]

Excellent work! Thank you.

[mbhrznr]

thanks for the clean up! 🙏