[3.12] gh-110367: Make regrtest --verbose3 compatible with --huntrleaks -jN (GH-111577) (#111589)

gh-110367: Make regrtest --verbose3 compatible with --huntrleaks -jN (GH-111577)

"./python -m test -j1 -R 3:3 --verbose3" now works as expected, since
run_single_test() does not replace sys.stdout with StringIO in this
case.
(cherry picked from commit d9a5530d2327efa1fe66a04d31b5c67e42dbcd9c)

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