proxy/grpcproxy: wait until register before Serve

It was fatal-ing with:

grpclog.Fatalf("grpc: Server.RegisterService after Server.Serve for %q", sd.ServiceName)

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

proxy/grpcproxy/cluster_test.go

@@ -107,7 +107,11 @@ func newClusterProxyServer(endpoints []string, t *testing.T) *clusterproxyTestSe
 	}
 	var opts []grpc.ServerOption
 	cts.server = grpc.NewServer(opts...)
-	go cts.server.Serve(cts.l)
+	ready := make(chan struct{})
+	go func() {
+		<-ready
+		cts.server.Serve(cts.l)
+	}()
 
 	// wait some time for free port 0 to be resolved
 	time.Sleep(500 * time.Millisecond)
@@ -116,6 +120,7 @@ func newClusterProxyServer(endpoints []string, t *testing.T) *clusterproxyTestSe
 	cts.cp, cts.donec = NewClusterProxy(client, cts.l.Addr().String(), "test-prefix")
 	cts.caddr = cts.l.Addr().String()
 	pb.RegisterClusterServer(cts.server, cts.cp)
+	close(ready)
 
 	return cts
 }