'``assertFailure`` fails if the deferred succeeds.'

def test_success(self):

    class Tests(AsyncTestCase, ):
        def test_success(self):
            return self.assertFailure(succeed(None), ValueError)
    result = run_test(Tests('test_success'))
    self.assertThat(result, has_results(failures=HasLength(1), tests_run=Equals(1)))
