[3.12] gh-110918: Fix side effects of regrtest test_match_tests() (GH-116718) (#116726)

gh-110918: Fix side effects of regrtest test_match_tests() (GH-116718)

test_match_tests now saves and restores patterns.

Add get_match_tests() function to libregrtest.filter.

Previously, running test_regrtest multiple times in a row only ran
tests once: "./python -m test test_regrtest -R 3:3.
(cherry picked from commit 612f1ec988314bc0bc42a1b908751950331e2ede)

Co-authored-by: Victor Stinner <vstinner@python.org>