Add missing unittest.main call

This should have been included in 48226a2f1f.

src/python/grpcio_tests/tests/unit/_invocation_defects_test.py

@@ -249,3 +249,7 @@ class InvocationDefectsTest(unittest.TestCase):
         with self.assertRaises(grpc.RpcError):
             for _ in range(test_constants.STREAM_LENGTH // 2 + 1):
                 next(response_iterator)
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)