[3.14] gh-146202: Create tmp_dir in regrtest worker (GH-146347) (#146349)

gh-146202: Create tmp_dir in regrtest worker (GH-146347)

Create tmp_dir in libregrtest.worker since the directory can be
different than the --tempdir directory.
(cherry picked from commit bcff99cb3f3b887a08c4f0ace1279ced38dd9e62)

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