[sdk-platform-java] Audit Gax's Unit Tests and clean up unnecessary tests

We are due for an audit and clean up of Gax's unit tests. The introduction of the showcase module that brings in a local server and we can look to either migrate certain [tests](https://github.com/googleapis/sdk-platform-java/blob/dfa9d2b7038f6e455a786dcf7b526bfb6ff8ece6/gax-java/gax-grpc/src/test/java/com/google/api/gax/grpc/GrpcDirectStreamControllerTest.java#L63) with a local server or remove them.