Distinguish WaitLeader to test more scenarios

## Enhancement Task

We need to distinguish the definition of a differentiated WaitLeader
- All pd recognized a leader (such as testing sync client).
- The follower does not need to recognize the leader while the leader has been elected at the moment.

Let's explain this test's error, And here is [detailed log](https://github.com/user-attachments/files/16092384/waitleader.txt)
```bash
2024-07-02T13:54:15.0492409Z --- FAIL: TestPrepareCheckerWithTransferLeader (76.09s)
2024-07-02T13:54:15.0492734Z region_syncer_test.go:289:
2024-07-02T13:54:15.0493797Z Error Trace: /home/runner/work/pd/pd/tests/server/region_syncer/region_syncer_test.go:289
2024-07-02T13:54:15.0494204Z Error: Not equal:
2024-07-02T13:54:15.0494642Z expected: "pd2"
2024-07-02T13:54:15.0495194Z actual : ""
2024-07-02T13:54:15.0495495Z
2024-07-02T13:54:15.0496104Z Diff:
2024-07-02T13:54:15.0496743Z --- Expected
2024-07-02T13:54:15.0497390Z +++ Actual
2024-07-02T13:54:15.0498020Z @@ -1 +1 @@
2024-07-02T13:54:15.0498413Z -pd2
2024-07-02T13:54:15.0498676Z +
2024-07-02T13:54:15.0499001Z Test: TestPrepareCheckerWithTransferLeader
2024-07-02T13:54:15.0499178Z FAIL
```

### Conclusion:
The [WaitLeader](https://github.com/tikv/pd/blob/02147789a144930f465a0142dd33da8f48c0d16b/tests/cluster.go#L649-L682) times out because all pd's do not all recognize the leader.

When [Checkleader](https://github.com/tikv/pd/blob/71490f72b4c57a70f4f5b4e3486018859f85189c/pkg/member/member.go#L228-L267)'s etcd get is too slow(> leader lease)
- follower was blocked, failing to recognize the current leader.
- The old leader exits leadership as lease time is reached within the follower blocking time.
- follower watch with revision can still fetch the leader exit event and then exit watch status.

The above event loop occurred for 50 seconds, causing the WaitLeader to time out.

### Detail explanation

#### multiple leaders changed
Due to the slow disk, the leader changed 8 times in the 50s!

```bash
[2024/07/02 13:53:01.139 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd1]
[2024/07/02 13:53:07.537 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd1]

!!! resign triggered by API !!!
2024-07-02T13:54:14.8419029Z [2024/07/02 13:53:11.565 +00:00] [INFO] [member.go:358] ["try to resign etcd leader to next pd-server"] [from=pd1] [to=]

[2024/07/02 13:53:12.651 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd2]
[2024/07/02 13:53:18.726 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd2]
[2024/07/02 13:53:24.777 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd2]

!!! resign pd-2 ->0 !!!
[2024/07/02 13:53:30.827 +00:00] [INFO] [member.go:358] ["try to resign etcd leader to next pd-server"] [from=pd2] [to=]
[2024/07/02 13:53:31.329 +00:00] [ERROR] [server.go:1718] ["campaign PD leader meets error due to etcd error"] [campaign-leader-name=pd2] [error="[PD:server:ErrLeaderFrequentlyChange]leader pd2 frequently changed, leader-key is [/pd/7387036149936305935/leader]"]

[2024/07/02 13:53:31.833 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd1] 1

!!! resign pd-1 -> 0 !!!
[2024/07/02 13:53:37.894 +00:00] [INFO] [member.go:358] ["try to resign etcd leader to next pd-server"] [from=pd1] [to=]
[2024/07/02 13:53:38.395 +00:00] [ERROR] [server.go:1718] ["campaign PD leader meets error due to etcd error"] [campaign-leader-name=pd1] [error="[PD:server:ErrLeaderFrequentlyChange]leader pd1 frequently changed, leader-key is [/pd/7387036149936305935/leader]"]

[2024/07/02 13:53:38.897 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd2] 2
[2024/07/02 13:53:44.934 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd2] 3
[2024/07/02 13:53:50.978 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd2] 4

!!! resign pd-2 -> 0 !!!
[2024/07/02 13:53:57.018 +00:00] [INFO] [member.go:358] ["try to resign etcd leader to next pd-server"] [from=pd2] [to=]
[2024/07/02 13:53:57.520 +00:00] [ERROR] [server.go:1718] ["campaign PD leader meets error due to etcd error"] [campaign-leader-name=pd2] [error="[PD:server:ErrLeaderFrequentlyChange]leader pd2 frequently changed, leader-key is [/pd/7387036149936305935/leader]"]
[2024/07/02 13:53:58.021 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd1] 5
```

#### failure WaitLeader

```bash
!!! pd2 was elected leader at 13:53:12.651 !!!
[2024/07/02 13:53:12.651 +00:00] [INFO] [server.go:1738] ["campaign PD leader ok"] [campaign-leader-name=pd2]

!!! pd1 skip campaigning stops at 13:53:12.479, at which point it goes to getPersistentLeader in CheckLeader, and then it takes 6s to get the leader and start watching !!!
[2024/07/02 13:53:12.278 +00:00] [INFO] [server.go:1700] ["skip campaigning of pd leader and check later"] [server-name=pd1] [etcd-leader-id=13430743067140231612] [member-id=15634440769998151696]
[2024/07/02 13:53:12.479 +00:00] [INFO] [server.go:1700] ["skip campaigning of pd leader and check later"] [server-name=pd1] [etcd-leader-id=13430743067140231612] [member-id=15634440769998151696]

!!! CheckLeader !!!
[2024/07/02 13:53:18.684 +00:00] [WARN] [etcdutil.go:153] ["kv gets too slow"] [request-key=/pd/7387036149936305935/leader] [cost=6.004986363s] []
[2024/07/02 13:53:18.725 +00:00] [INFO] [server.go:1669] ["start to watch pd leader"] [pd-leader="name:\"pd2\" member_id:13430743067140231612 peer_urls:\"http://127.0.0.1:38109\" client_urls:\"http://127.0.0.1:40999\" "]

!!! pd1 watch found the leader changed after only 150ms, resulting in the WaitLeader not being successful !!!
[2024/07/02 13:53:18.862 +00:00] [INFO] [leadership.go:388] ["current leadership is deleted"] [revision=57] [leader-key=/pd/7387036149936305935/leader] [purpose="leader election"]
[2024/07/02 13:53:19.728 +00:00] [INFO] [server.go:1673] ["pd leader has changed, try to re-campaign a pd leader"]
```