test_pty started failing on Windows, but if and only if test___all__ was

test_pty started failing on Windows, but if and only if test___all__ was
run first.  Indirectly due to Skip adding check_all("pty") to test___all__:
that caused the expected ImportError due to pty.py trying to import the
non-existent FCNTL to get handled by test___all__, leaving a partial
module object for pty in sys.modules, which caused the later import of
pty via test_pty to succeed.  Then test_tpy died with an AttributeError,
due to trying to access attributes of pty that didn't exist.  regrtest
viewed that as a failure rather than the appropriate "test skipped".
Fixed by deleting partial module objects in test___all__ when test___all__
handles an ImportError.