image testing and comparison revamping

[jGaboardi]

image testing and comparison revamping

This PR:

• resolves #266
• resolves #264
• resolves #262
• resolves #261
• resolves #260
• resolves #259

---

• gets CI green
• omits text/font from image comparison
• negates the need to skip certain MPL versions
• CI artifacts can be downloaded from the associated action run summary
  • e.g. here for action run 15655277902 triggered by commit 50531a5

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.0%. Comparing base (741ca5d) to head (f0d4005).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #265     +/-   ##
=======================================
+ Coverage   87.7%   90.0%   +2.3%     
=======================================
  Files         10      10             
  Lines       1170    1170             
=======================================
+ Hits        1026    1053     +27     
+ Misses       144     117     -27     

see 2 files with indirect coverage changes

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