TestClientWatchWithRevision is unstable

## Flaky Test

### Which jobs are failing

TestGlobalConfigTestSuite
### CI link

### Reason for failure (if possible)
```
--- FAIL: TestGlobalConfigTestSuite (5.14s)
2023-02-17T03:13:58.0444036Z --- FAIL: TestGlobalConfigTestSuite/TestClientWatchWithRevision (1.06s)
2023-02-17T03:13:58.0444283Z global_config_test.go:293:
2023-02-17T03:13:58.0444834Z Error Trace: /home/runner/work/pd/pd/tests/client/global_config_test.go:293
2023-02-17T03:13:58.0445131Z Error: Not equal:
2023-02-17T03:13:58.0445435Z expected: 28
2023-02-17T03:13:58.0445729Z actual : 29
2023-02-17T03:13:58.0446939Z Test: TestGlobalConfigTestSuite/TestClientWatchWithRevision
2023-02-17T03:13:58.0447022Z FAIL
2023-02-17T03:13:58.0447545Z [2023/02/17 03:13:56.633 +00:00] [WARN] [grpclog.go:60] ["grpc: addrConn.createTransport failed to connect to {127.0.0.1:46139 0 }. Err :connection error: desc = \"transport: Error while dialing dial tcp 127.0.0.1:46139: connect: connection refused\". Reconnecting..."]
```

### Anything else