Disable sending activation email also when doing external_auth and setting BYPASS_ACTIVATION_EMAIL_FOR_EXTAUTH=True

See #4256
I don't think you're mocking the right thing here. Can you write a second test that is identical to this one, except that `settings.FEATURES['BYPASS_ACTIVATION_EMAIL_FOR_EXTAUTH']=False`, and be sure that the `send_mail` mock *is* called?