test: stabilize TestAddIndexMergeConflictWithPessimistic (pingcap#40975)

close pingcap#40939

Author: tangenta

ddl/indexmergetest/BUILD.bazel

@@ -8,6 +8,7 @@ go_test(
         "merge_test.go",
     ],
     flaky = True,
+    race = "on",
     shard_count = 4,
     deps = [
         "//config",

ddl/indexmergetest/merge_test.go

@@ -478,6 +478,7 @@ func TestAddIndexMergeConflictWithPessimistic(t *testing.T) {
 	callback := &callback.TestDDLCallback{Do: dom}
 
 	runPessimisticTxn := false
+	afterPessDML := make(chan struct{}, 1)
 	callback.OnJobRunBeforeExported = func(job *model.Job) {
 		if t.Failed() {
 			return
@@ -500,6 +501,7 @@ func TestAddIndexMergeConflictWithPessimistic(t *testing.T) {
 			assert.NoError(t, err)
 			_, err = tk2.Exec("update t set a = 3 where id = 1;")
 			assert.NoError(t, err)
+			afterPessDML <- struct{}{}
 		}
 	}
 	dom.DDL().SetHook(callback)
@@ -515,6 +517,7 @@ func TestAddIndexMergeConflictWithPessimistic(t *testing.T) {
 	case <-afterCommit:
 		require.Fail(t, "should be blocked by the pessimistic txn")
 	}
+	<-afterPessDML
 	tk2.MustExec("rollback;")
 	<-afterCommit
 	dom.DDL().SetHook(originHook)