Big cleanup of test case reduction utilities

[c42f]

• Text-based test reduction moved into test_utils
• Remove some obsolete utils
• Rename reduction utils to something more sensible
• List actual reduced failures in package parsing tests

[codecov[bot]]

Codecov Report

Merging #250 (40d113e) into main (1515d8f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #250   +/-   ##
=======================================
  Coverage   96.47%   96.47%           
=======================================
  Files          15       15           
  Lines        3919     3919           
=======================================
  Hits         3781     3781           
  Misses        138      138           

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