test/cluster_test: make TestRegionStatistics more stable (#8320)

close #8319

Because this test has only 2 pd's and five ResignLeader times, whenever there is a network or disk problem will cause the lease to expire, and result in one more leader resigning.
And then there is a high probability of being recognized as frequent campaign times to transfer leader failed

Signed-off-by: husharp <jinhao.hu@pingcap.com>

tests/server/cluster/cluster_test.go

@@ -186,7 +186,7 @@ func TestRegionStatistics(t *testing.T) {
 	re := require.New(t)
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-	tc, err := tests.NewTestCluster(ctx, 2)
+	tc, err := tests.NewTestCluster(ctx, 3)
 	defer tc.Destroy()
 	re.NoError(err)
 
@@ -223,9 +223,9 @@ func TestRegionStatistics(t *testing.T) {
 	time.Sleep(1000 * time.Millisecond)
 
 	leaderServer.ResignLeader()
-	newLeaderName := tc.WaitLeader()
-	re.NotEqual(newLeaderName, leaderName)
+	re.NotEqual(tc.WaitLeader(), leaderName)
 	leaderServer = tc.GetLeaderServer()
+	leaderName = leaderServer.GetServer().Name()
 	rc = leaderServer.GetRaftCluster()
 	r := rc.GetRegion(region.Id)
 	re.NotNil(r)
@@ -238,9 +238,9 @@ func TestRegionStatistics(t *testing.T) {
 	re.Len(regions, 1)
 
 	leaderServer.ResignLeader()
-	newLeaderName = tc.WaitLeader()
-	re.Equal(newLeaderName, leaderName)
+	re.NotEqual(tc.WaitLeader(), leaderName)
 	leaderServer = tc.GetLeaderServer()
+	leaderName = leaderServer.GetServer().Name()
 	rc = leaderServer.GetRaftCluster()
 	re.NotNil(r)
 	re.True(r.LoadedFromStorage() || r.LoadedFromSync())
@@ -255,13 +255,12 @@ func TestRegionStatistics(t *testing.T) {
 	re.False(r.LoadedFromStorage() && r.LoadedFromSync())
 
 	leaderServer.ResignLeader()
-	newLeaderName = tc.WaitLeader()
-	re.NotEqual(newLeaderName, leaderName)
-	leaderServer.ResignLeader()
-	newLeaderName = tc.WaitLeader()
-	re.Equal(newLeaderName, leaderName)
+	re.NotEqual(tc.WaitLeader(), leaderName)
 	leaderServer = tc.GetLeaderServer()
-	rc = leaderServer.GetRaftCluster()
+	leaderName = leaderServer.GetServer().Name()
+	leaderServer.ResignLeader()
+	re.NotEqual(tc.WaitLeader(), leaderName)
+	rc = tc.GetLeaderServer().GetRaftCluster()
 	r = rc.GetRegion(region.Id)
 	re.NotNil(r)
 	re.False(r.LoadedFromStorage() && r.LoadedFromSync())