gh-93096: fix test_mimetypes.test_invocation_error tests on iOS ARM64

[picnixz]

• Issue: Decide the fate of undocumented script behavior of some modules #93096

[picnixz]

On iOS, mocking didn't seem to work correctly (and I don't know why) so we'll just make something even more unknown. If bar_ext is a registered extension, then I think the system is really weird.

[picnixz]

And I forgot to print the help correctly...

[picnixz]

Hum. Interestingly, on main python -m mimetypes actually exits before printing the help. So technically... it's not even needed:

$ python -m mimetypes
usage: python -m mimetypes [-h] [-e] [-l] type [type ...]
python -m mimetypes: error: the following arguments are required: type

The (incorrect) return parser.format_help() is not even reached because _parse_args() is already raising SystemExit (this is because the argument is required!)

[picnixz]

Great, the test suite now passes on iOS (still running):

0:08:04 load avg: 31.35 [278/486] test_mimetypes
0:08:04 load avg: 44.62 [278/486] test_mimetypes passed