TestReloadLabel is flaky

## Flaky Test

### Which jobs are failing

```
--- FAIL: TestReloadLabel (339.90s)
panic: exit status 2 [recovered]
panic: exit status 2

goroutine 357 [running]:
testing.tRunner.func1.2({0x1ad2aa0, 0xc00055bb40})
/usr/local/go/src/testing/testing.go:1545 +0x3f7
testing.tRunner.func1()
/usr/local/go/src/testing/testing.go:1548 +0x716
panic({0x1ad2aa0?, 0xc00055bb40?})
/usr/local/go/src/runtime/panic.go:920 +0x270
github.com/tikv/pd/tests/integrations/realcluster.restartTiUP()
/home/jenkins/agent/workspace/tikv/pd/pull_integration_realcluster_test/pd/tests/integrations/realcluster/reboot_pd_test.go:31 +0xb2
github.com/tikv/pd/tests/integrations/realcluster.TestReloadLabel(0xc0001a3d40)
/home/jenkins/agent/workspace/tikv/pd/pull_integration_realcluster_test/pd/tests/integrations/realcluster/reboot_pd_test.go:75 +0x4d8
testing.tRunner(0xc0001a3d40, 0x1bcf890)
/usr/local/go/src/testing/testing.go:1595 +0x262
created by testing.(*T).Run in goroutine 1
/usr/local/go/src/testing/testing.go:1648 +0x846
FAIL github.com/tikv/pd/tests/integrations/realcluster 340.028s
````

### CI link

https://do.pingcap.net/jenkins/blue/organizations/jenkins/tikv%2Fpd%2Fpull_integration_realcluster_test/detail/pull_integration_realcluster_test/290/pipeline/

### Reason for failure (if possible)

### Anything else