[3.15] gh-83386: Enable test_hang_gh83386 for ProcessPoolExecutor (GH-152976) (#152982)

[3.15] gh-83386: Enable test_hang_gh83386 for ProcessPoolExecutor (GH-152976) (#152982)

gh-83386: Enable test_hang_gh83386 for ProcessPoolExecutor (GH-152976)

The hang this test guards against (interpreter exit after
shutdown(wait=False) with running futures) was fixed for
ProcessPoolExecutor by the executor management rewrite years ago, but
the test still skipped it citing the issue. The skip also hid a latent
NameError in the subprocess template, which only selects a start method
in the process pool variants; rewrite it to use the same
mp_context=get_context() shape as the sibling templates. Remove a stale
comment claiming ProcessPoolExecutor often hangs with wait=False.
(cherry picked from commit 548c7314138b85cab3b945a1ce05440dd836a2ea)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>