LiveServerTestCase.setUpClass() doesn't clean up fully on failure

[Chris Jerdonek]

While working on #32417, I noticed in this comment that LiveServerTestCase.setUpClass() doesn't clean up all the way if setUpClass() errors out.

Specifically, LiveServerTestCase.setUpClass() ​looks like this:

@classmethod
def setUpClass(cls):
    super().setUpClass()
    ...
    cls._live_server_modified_settings = modify_settings(
        ALLOWED_HOSTS={'append': cls.allowed_host},
    )
    cls._live_server_modified_settings.enable()
    ...
    if cls.server_thread.error:
        # Clean up behind ourselves, since tearDownClass won't get called in
        # case of errors.
        cls._tearDownClassInternal()
        raise cls.server_thread.error

But _tearDownClassInternal() ​doesn't undo the call to cls._live_server_modified_settings.enable().

I will post a PR shortly.