DiffErrString returns an empty diff string if the 'got' error message contains the 'want' string.

FakeGRPCServer creates and registers a fake grpc server for testing.

IsIntegration checks env var TEST_INTEGRATION and consider that we're in an integration test if it's set to true.

SkipIfNotIntegration skips the test if [IsIntegration] returns false.

RegisterFunc is the callback to register a fake gRPC service to the given server.