Add a GnuTests workflow that, on push and PR, fetches the GNU grep release
tarball, builds the Rust grep binary, runs util/run-gnu-testsuite.sh, and
uploads the JSON results. An aggregate job compares the run against the
reference summary from the default branch (util/compare_test_results.py,
borrowed from ../sed) and fails only on new, non-intermittent regressions;
known-flaky tests are listed in .github/workflows/ignore-intermittent.txt.
