TestSchedulerDiagnostic is unstable

## Flaky Test
TestSchedulerTestSuite/TestSchedulerDiagnostic

### Which jobs are failing
```
--- FAIL: TestSchedulerTestSuite/TestScheduler (44.04s)
testutil.go:64:
Error Trace: /home/runner/work/pd/pd/pkg/utils/testutil/testutil.go:64
/home/runner/work/pd/pd/tests/pdctl/scheduler/scheduler_test.go:426
/home/runner/work/pd/pd/tests/pdctl/scheduler/scheduler_test.go:508
/home/runner/work/pd/pd/tests/testutil.go:309
/home/runner/work/pd/pd/tests/testutil.go:270
/home/runner/work/pd/pd/tests/pdctl/scheduler/scheduler_test.go:99
Error: Condition never satisfied
Test: TestSchedulerTestSuite/TestScheduler
testutil.go:275: start test TestSchedulerDiagnostic in pd mode
--- FAIL: TestSchedulerTestSuite/TestSchedulerDiagnostic (20.03s)
testutil.go:64:
Error Trace: /home/runner/work/pd/pd/pkg/utils/testutil/testutil.go:64
/home/runner/work/pd/pd/tests/pdctl/scheduler/scheduler_test.go:644
/home/runner/work/pd/pd/tests/pdctl/scheduler/scheduler_test.go:681
/home/runner/work/pd/pd/tests/testutil.go:279
/home/runner/work/pd/pd/tests/testutil.go:269
/home/runner/work/pd/pd/tests/pdctl/scheduler/scheduler_test.go:634
Error: Condition never satisfied
Test: TestSchedulerTestSuite/TestSchedulerDiagnostic
```

### CI link
https://github.com/tikv/pd/actions/runs/7296294204/job/19883901919

### Reason for failure (if possible)

### Anything else