def test_iterateNestedTestSuite(self):
        """
        L{_iterateTests} returns tests that are in nested test suites.
        """
        test = self.TestCase()
        suite = runner.TestSuite([runner.TestSuite([test])])
        self.assertEqual([test], list(_iterateTests(suite)))

        L{_iterateTests} returns tests that are in nested test suites.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/test_tests.py