Fix the goroutine leak in StubServer

---

(If this PR fixes a github issue, please add `Fixes #<xyz>`)

Fixes etcd-io#13023

(or if this PR is one task of a github issue, please add `Master Issue: #<xyz>`
to link to the master issue)

Master Issue: etcd-io#13023

*Motivation*

Currently, StubServer can stop itself (by Stop()) before it start its
grpc server (created by Start()). This race condition may lead to a
goroutine leak mentioned by etcd-io#13023.

*Modifications*

This PR add a channel to force the Stop() started after Start()

*Verify this change*

Please pick either of following options.

- This change is already covered by existing tests, such as *(please describe tests)*.

Test/TestEtcdGrpcResolver

pkg/grpc_testing/stub_server.go

@@ -26,10 +26,14 @@ type StubServer struct {
 	s *grpc.Server
 
 	cleanups []func() // Lambdas executed in Stop(); populated by Start().
+	started chan struct{}
 }
 
 func New(testService testpb.TestServiceServer) *StubServer {
-	return &StubServer{testService: testService}
+	return &StubServer{
+		testService: testService,
+		started: make(chan struct{}),
+	}
 }
 
 // Start starts the server and creates a client connected to it.
@@ -50,7 +54,10 @@ func (ss *StubServer) Start(sopts []grpc.ServerOption, dopts ...grpc.DialOption)
 
 	s := grpc.NewServer(sopts...)
 	testpb.RegisterTestServiceServer(s, ss.testService)
-	go s.Serve(lis)
+	go func() {
+		close(ss.started)
+		s.Serve(lis)
+	}()
 	ss.cleanups = append(ss.cleanups, s.Stop)
 	ss.s = s
 
@@ -59,6 +66,7 @@ func (ss *StubServer) Start(sopts []grpc.ServerOption, dopts ...grpc.DialOption)
 
 // Stop stops ss and cleans up all resources it consumed.
 func (ss *StubServer) Stop() {
+	<-ss.started
 	for i := len(ss.cleanups) - 1; i >= 0; i-- {
 		ss.cleanups[i]()
 	}