'--timeout' option requires faulthandler.dump_traceback_later).

If bots fail due to using this flag, the buildbot scripts have to be modified
to omit it. Regrtest ignores it anyway.

parsing.  Added more tests for regrtest's argument parsing.

This fixes a regression introduced in the commit for issue #15302, which
switched regrtest from getopt to argparse.

This is the first step in refactoring regrtest to use argparse.  The
regrtest module's main() function still expects a getopt-style return
value rather than an argparse.Namespace instance.