Only run tests when the PR changes test code

[mdekstrand]

This adds a check to see if we have actually changed any code, and if not it skips tests to reduce overhead when we're working on doc tweaks and such.

tests: force

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.98%. Comparing base (abeb934) to head (9525e7d).
Report is 561 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #433   +/-   ##
=======================================
  Coverage   90.98%   90.98%           
=======================================
  Files          47       47           
  Lines        3207     3207           
=======================================
  Hits         2918     2918           
  Misses        289      289           

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