Correct tests.json

src/python/grpcio_tests/tests/tests.json

@@ -70,7 +70,7 @@
   "unit._server_test.ServerTest",
   "unit._server_wait_for_termination_test.ServerWaitForTerminationTest",
   "unit._session_cache_test.SSLSessionCacheTest",
-  "unit._simple_stubs_test.SimpleStubsTest",
+  "unit.py3_only._simple_stubs_test.SimpleStubsTest",
   "unit._signal_handling_test.SignalHandlingTest",
   "unit._version_test.VersionTest",
   "unit.beta._beta_features_test.BetaFeaturesTest",