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

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.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>