ensure unit tests work for GAPICs using transport=rest

[vchudnov-g]

I can locally run the generated unit tests for the texttospeech GAPIC generated with transport=grpc, and they pass. When transport=rest, those tests fail. We should ensure the tests generated for the clients can run and pass.

Having the tests running is a blocker for releasing the transport=rest functionality.

[vchudnov-g]

Note that we should run the tests when numeric enums are on and off, to make sure both cases work.