ddl: fix data race in TestTiFlashBatchRateLimiter (#33006)

close #33005

ddl/ddl_tiflash_test.go

@@ -22,6 +22,7 @@ import (
 	"context"
 	"fmt"
 	"math"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -887,12 +888,17 @@ func TestTiFlashBatchRateLimiter(t *testing.T) {
 
 	// Retrigger, but close session before the whole job ends.
 	var wg util.WaitGroupWrapper
+	var mu sync.Mutex
 	wg.Run(func() {
 		time.Sleep(time.Millisecond * 20)
+		mu.Lock()
+		defer mu.Unlock()
 		tk.Session().Close()
 		logutil.BgLogger().Info("session closed")
 	})
+	mu.Lock()
 	timeOut, err = execWithTimeout(t, tk, time.Second*2, "alter database tiflash_ddl_limit set tiflash replica 1")
+	mu.Unlock()
 	require.NoError(t, err)
 	require.False(t, timeOut)
 	check(5, 5)