def METHOD_NAME(self):
    retired_user = CommCareUser.create(self.domain, 'retired@test-domain.commcarehq.org', 'abc123', None, None)
    self.addCleanup(retired_user.delete, self.domain, None)
    retired_user.retire(self.domain, None)
    with self.assertRaises(ValidationError) as cm:
        validate_mobile_username('retired@test-domain.commcarehq.org', self.domain)
    self.assertEqual(cm.exception.message,
                     "Username 'retired@test-domain.commcarehq.org' is already taken or reserved.")