RegisterServiceServerOption returns a ServerOption that will run f() in Start or StartServer with the grpc.Server created before serving.

StartTestService spins up a stub server exposing the TestService on a local port.

StubServer is a server that is easy to customize within individual test cases.

GRPCServer is an interface that groups methods implemented by a grpc.Server or an xds.GRPCServer that are used by the StubServer.