bpo-36915: regrtest always remove tempdir of worker processes (GH-13312)

bpo-36915: regrtest always remove tempdir of worker processes (GH-13312)

When using multiprocessing (-jN option), worker processes now create
their temporary directory inside the temporary directory of the
main process. So the main process is able to remove temporary
directories of worker processes even if they crash or when they are
killed by regrtest on KeyboardInterrupt (CTRL+c).

Rework also how multiprocessing arguments are parsed in main.py.
(cherry picked from commit 3c93153f7db5dd9b06f229e61978fd9199b3c097)

Co-authored-by: Victor Stinner <vstinner@redhat.com>