ensure we have test coverage for gRPC bidi streaming

[This comment thread](https://github.com/googleapis/gapic-generator-python/commit/c4408075f8cc8a270af4a818ea7aafc0c2ad6a7e#r146008230) highlights that some unknown time ago, code that was apparently meant to cover several cases including bi-directional streaming was moved into sections of macro files that were only generated `{% if not method.client_streaming %}`. This raises the possibility that the corresponding bidi code might have had its test coverage accidentally removed. We should check whether that is indeed the case (I suggest starting with git blame to see when the code made it into `test_macros.j2` from where), and if so, fix it.