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.

Lib/test/test_regrtest.py

@@ -1153,7 +1153,6 @@ class ArgsTestCase(BaseTestCase):
                                   env_changed=[testname],
                                   fail_env_changed=True)
 
-    @unittest.skipIf(True, 'bpo-37531, bpo-38207: test hangs randomly')
     def test_multiprocessing_timeout(self):
         code = textwrap.dedent(r"""
             import time