'Verify that non-existent users are not added.'

def test_only_unknown(self):

    usernames = ['unknown_user{}'.format(i) for i in range(3)]
    response_dict = self.request_add_users_to_cohort(','.join(usernames), self.cohort1, self.course)
    self.verify_added_users_to_cohort(response_dict, self.cohort1, self.course, expected_added=[], expected_changed=[], expected_present=[], expected_preassigned=[], expected_unknown=usernames, expected_invalid=[])
