ci: per-test GNU/bfs comparison and comment, like uutils sed/grep

[sylvestre]

The old comparison only commented when the PASS/FAIL/ERROR/SKIP counts changed vs main, so it was blind to net-zero changes (one test fixed, one broken). diff-gnu.sh also compared against an unextracted (empty) baseline, flagging every current failure as a regression, and the published baseline could be stale.

Adopt the sed/grep design:

• build-gnu.sh / build-bfs.sh now emit a per-test JSON summary (findutils-gnu-full-result.json / bfs-full-result.json) listing every test's name + PASS/FAIL/SKIP, via the new gnu_json_result.py / bfs_json_result.py.
• compare_test_results.py diffs the set of failing test names against a reference, writing a human-readable result file (empty when nothing changed) and exiting non-zero only on new, non-intermittent failures.
• compat.yml fetches the reference *-full-result.json from the default branch's latest run (dawidd6/action-download-artifact) and uploads a comparison artifact per suite.
• comment.yml posts a PR comment whenever the comparison is non-empty, i.e. on any per-test improvement or regression regardless of net count.
• ignore-intermittent.txt lists known-flaky tests, reported but not failing the job.

Remove the superseded compare_gnu_result.py / compare_bfs_result.py / diff-gnu.sh / diff-bfs.sh.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.86%. Comparing base (5398003) to head (4ae0f1a).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #726   +/-   ##
=======================================
  Coverage   91.85%   91.86%           
=======================================
  Files          35       35           
  Lines        7149     7155    +6     
  Branches      375      375           
=======================================
+ Hits         6567     6573    +6     
  Misses        437      437           
  Partials      145      145           

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[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

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_{Comparing sylvestre:ci-per-test-comparison (4ae0f1a) with main (001966f)}