TestOperatorTestSuite/TestTransferRegionWithPlacementRule is flaky

## Flaky Test
### Which jobs are failing
```text
=== NAME TestOperatorTestSuite/TestTransferRegionWithPlacementRule
api_check.go:42:
Error Trace: /home/prow/go/src/github.com/tikv/pd/pkg/utils/testutil/api_check.go:42
/home/prow/go/src/github.com/tikv/pd/pkg/utils/testutil/api_check.go:175
/home/prow/go/src/github.com/tikv/pd/pkg/utils/testutil/api_check.go:114
/home/prow/go/src/github.com/tikv/pd/tests/server/api/operator_test.go:437
/home/prow/go/src/github.com/tikv/pd/tests/testutil.go:607
/home/prow/go/src/github.com/tikv/pd/tests/testutil.go:426
/home/prow/go/src/github.com/tikv/pd/tests/testutil.go:398
/home/prow/go/src/github.com/tikv/pd/tests/server/api/operator_test.go:213
Error: Should not be: 200
Test: TestOperatorTestSuite/TestTransferRegionWithPlacementRule
```
### CI link
https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/tikv_pd/10212/pull-unit-test-next-gen-2/2032357808665530368
https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/tikv_pd/10317/pull-unit-test-next-gen-2/2032369465567481856
https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/tikv_pd/10317/pull-unit-test-next-gen-2/2032371755678437376
### Reason for failure (if possible)

### Anything else
test=TestOperatorTestSuite/TestTransferRegionWithPlacementRule signatures=UNKNOWN_FAILURE,DATA_RACE,CONDITION_NEVER_SATISFIED