Fix Data Race in TestServerRegisterTestSuite

==================
WARNING: DATA RACE
Read at 0x00c000406fd0 by goroutine 718:
  github.com/tikv/pd/pkg/mcs/tso/server.(*Server).stopHTTPServer.func1()
      /home/runner/work/pd/pd/pkg/mcs/tso/server/server.go:465 +0xde

Previous write at 0x00c000406fd0 by goroutine 508:
  github.com/tikv/pd/pkg/mcs/tso/server.(*Server).startGRPCAndHTTPServers()
      /home/runner/work/pd/pd/pkg/mcs/tso/server/server.go:436 +0x907
  github.com/tikv/pd/pkg/mcs/tso/server.(*Server).startServer.func1()
      /home/runner/work/pd/pd/pkg/mcs/tso/server/server.go:561 +0x58

Goroutine 718 (running) created at:
  github.com/tikv/pd/pkg/mcs/tso/server.(*Server).stopHTTPServer()
      /home/runner/work/pd/pd/pkg/mcs/tso/server/server.go:462 +0x24d
  github.com/tikv/pd/pkg/mcs/tso/server.(*Server).Close()
      /home/runner/work/pd/pd/pkg/mcs/tso/server/server.go:193 +0x13c
  github.com/tikv/pd/tests/integrations/mcs/discovery_test.(*serverRegisterTestSuite).checkServerPrimaryChange()
      /home/runner/work/pd/pd/tests/integrations/mcs/discovery/register_test.go:139 +0x37b
  github.com/tikv/pd/tests/integrations/mcs/discovery_test.(*serverRegisterTestSuite).TestServerPrimaryChange()
      /home/runner/work/pd/pd/tests/integrations/mcs/discovery/register_test.go:116 +0x3d

Signed-off-by: Bin Shi <binshi.bing@gmail.com>

pkg/mcs/tso/server/server.go

@@ -414,7 +414,7 @@ func (s *Server) startHTTPServer(l net.Listener) {
 	}
 }
 
-func (s *Server) startGRPCAndHTTPServers(l net.Listener) {
+func (s *Server) startGRPCAndHTTPServers(serverReadyChan chan<- struct{}, l net.Listener) {
 	defer logutil.LogPanic()
 	defer s.serverLoopWg.Done()
 
@@ -441,6 +441,7 @@ func (s *Server) startGRPCAndHTTPServers(l net.Listener) {
 	s.serverLoopWg.Add(1)
 	go s.startHTTPServer(s.httpListener)
 
+	serverReadyChan <- struct{}{}
 	if err := mux.Serve(); err != nil {
 		if s.IsClosed() {
 			log.Info("[tso] mux stopped serving", errs.ZapError(err))
@@ -461,14 +462,14 @@ func (s *Server) stopHTTPServer() {
 	ch := make(chan struct{})
 	go func() {
 		defer close(ch)
-		log.Warn("[tso] http server graceful shutdown timeout, forcing close")
 		s.httpServer.Shutdown(ctx)
 	}()
 
 	select {
 	case <-ch:
 	case <-ctx.Done():
 		// Took too long, manually close open transports
+		log.Warn("[tso] http server graceful shutdown timeout, forcing close")
 		s.httpServer.Close()
 		// concurrent Graceful Shutdown should be interrupted
 		<-ch
@@ -557,8 +558,11 @@ func (s *Server) startServer() (err error) {
 		return err
 	}
 
+	serverReadyChan := make(chan struct{})
+	defer close(serverReadyChan)
 	s.serverLoopWg.Add(1)
-	go s.startGRPCAndHTTPServers(s.muxListener)
+	go s.startGRPCAndHTTPServers(serverReadyChan, s.muxListener)
+	<-serverReadyChan
 
 	// Run callbacks
 	log.Info("triggering the start callback functions")