feat: add GNU testsuite tracking workflows

[pocopepe]

Closes #64

took inspiration from how sed does it and adapted it for diffutils, open to feedback if I've missed anything

GnuTests.yml runs the upstream gnu diffutils test suite on every push and pr, compares results against main branch to catch regressions
GnuComment.yml posts a comparison summary as a pr comment, updating it instead of spamming new ones
util/compare_test_results.py comparison logic, exits 1 on new non-intermittent failures
Removed the old gnu-testsuite from ci.yml

[pocopepe]

We'd need a tracking repo like sed-tracking wouldn't we? we're already producing the test-summary here