Is your feature request related to a problem? Please describe.

I'd like to use GetPolicy() and SetPolicy() in the tests that rely on the pstest.Server. For this, I would need to register my FakeIAMPolicyServer into the grpc.Server instance. Currently the instance is hidden behind unexposed variables.

Describe the solution you'd like

I would like to add new method with callback function to initialise the pstest.Server:

func NewServerWithFakes(port int, registerFakes func(*grpc.Server), opts ...ServerReactorOption) *Server

The existing NewServer and NewServerWithPort would use internally empty callback for backwards compatiblity.

The server registration process would change like this:

	pb.RegisterPublisherServer(srv.Gsrv, &s.GServer)
	pb.RegisterSubscriberServer(srv.Gsrv, &s.GServer)
	pb.RegisterSchemaServiceServer(srv.Gsrv, &s.GServer)

	// register user provided fake servers, such as IAMPolicyServer
	registerFakes(srv.Gsrv)
	srv.Start()

The new method would be used for example like this:

	srv := pstest.NewServerWithFakes(
		0,
		func (gsrv *grpc.Server) {
			// register one or more additional fake server
			iampb.RegisterIAMPolicyServer(gsrv, &FakeIAMPolicyServer{})
		},
	)

Describe alternatives you've considered
The srv in the pstest.Server could be exposed. But then we would need new method that creates new server without calling the srv.Start(). Registering additional fakes is only possible before the Gsrv.Serve() is called.