TestGetTSOImmediately failed

## Flaky Test

### Which jobs are failing
```
2023-07-12T16:54:44.1099413Z testutil.go:178:
2023-07-12T16:54:44.1099865Z Error Trace: /home/runner/work/pd/pd/tests/server/apiv2/handlers/testutil.go:178
2023-07-12T16:54:44.1100997Z /home/runner/work/pd/pd/tests/integrations/mcs/tso/keyspace_group_manager_test.go:360
2023-07-12T16:54:44.1102034Z /opt/hostedtoolcache/go/1.20.1/x64/src/runtime/asm_amd64.s:1598
2023-07-12T16:54:44.1102228Z Error: Not equal:
2023-07-12T16:54:44.1102493Z expected: 200
2023-07-12T16:54:44.1102751Z actual : 500
2023-07-12T16:54:44.1103080Z Test: TestGetTSOImmediately
2023-07-12T16:54:44.1103571Z Messages: "[PD:etcd:ErrEtcdTxnConflict]etcd transaction failed, conflicted and rolled back"
```
### CI link

https://github.com/tikv/pd/actions/runs/5534201396/jobs/10098828392?pr=6577
### Reason for failure (if possible)

### Anything else