bpo-37531: regrtest ignores output on timeout (GH-16659)

bpo-37531: regrtest ignores output on timeout (GH-16659)

bpo-37531, bpo-38207: On timeout, regrtest no longer attempts to call
`popen.communicate() again: it can hang until all child processes
using stdout and stderr pipes completes. Kill the worker process and
ignores its output.

Reenable test_regrtest.test_multiprocessing_timeout().

bpo-37531: Change also the faulthandler timeout of the main process
from 1 minute to 5 minutes, for Python slowest buildbots.
(cherry picked from commit 0ec618af98ac250a91ee9c91f8569e6df6772758)

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