pd-ut test processes can allocate duplicate ports

## Flaky Test

### Which jobs are failing

TestForwardTestSuite

```
testutil.go:622:
Error Trace: /home/prow/go/src/github.com/tikv/pd/tests/testutil.go:622
/home/prow/go/src/github.com/tikv/pd/tests/testutil.go:591
/home/prow/go/src/github.com/tikv/pd/tests/testutil.go:412
/home/prow/go/src/github.com/tikv/pd/tests/server/api/api_test.go:1027
/home/prow/go/pkg/mod/github.com/stretchr/testify@v1.10.0/suite/suite.go:157
/home/prow/go/src/github.com/tikv/pd/tests/server/api/api_test.go:1018
Error: Received unexpected error:
Etcd cluster ID mismatch, expect 9210147299642074314, got 6474684432105739760
github.com/tikv/pd/pkg/utils/etcdutil.CheckClusterID
/home/prow/go/src/github.com/tikv/pd/pkg/utils/etcdutil/etcdutil.go:103
github.com/tikv/pd/server.(*Server).startEtcd
/home/prow/go/src/github.com/tikv/pd/server/server.go:352
github.com/tikv/pd/server.(*Server).Run
/home/prow/go/src/github.com/tikv/pd/server/server.go:627
github.com/tikv/pd/tests.(*TestServer).Run
/home/prow/go/src/github.com/tikv/pd/tests/cluster.go:149
github.com/tikv/pd/tests.RunServer.func1
/home/prow/go/src/github.com/tikv/pd/tests/cluster.go:652
runtime.goexit
/usr/local/go/src/runtime/asm_amd64.s:1693
github.com/tikv/pd/tests.RunServers
/home/prow/go/src/github.com/tikv/pd/tests/cluster.go:664
github.com/tikv/pd/tests.(*TestCluster).runInitialServersWithRetry
/home/prow/go/src/github.com/tikv/pd/tests/cluster.go:687
github.com/tikv/pd/tests.(*TestCluster).RunInitialServers
/home/prow/go/src/github.com/tikv/pd/tests/cluster.go:672
github.com/tikv/pd/tests.(*SchedulingTestEnvironment).startCluster
/home/prow/go/src/github.com/tikv/pd/tests/testutil.go:621
github.com/tikv/pd/tests.(*SchedulingTestEnvironment).runFuncInNonMicroserviceEnv
/home/prow/go/src/github.com/tikv/pd/tests/testutil.go:591
github.com/tikv/pd/tests.(*SchedulingTestEnvironment).RunFunc
/home/prow/go/src/github.com/tikv/pd/tests/testutil.go:412
github.com/tikv/pd/tests/server/api.(*forwardTestSuite).SetupSuite
/home/prow/go/src/github.com/tikv/pd/tests/server/api/api_test.go:1027
github.com/stretchr/testify/suite.Run
/home/prow/go/pkg/mod/github.com/stretchr/testify@v1.10.0/suite/suite.go:157
github.com/tikv/pd/tests/server/api.TestForwardTestSuite
/home/prow/go/src/github.com/tikv/pd/tests/server/api/api_test.go:1018
testing.tRunner
/usr/local/go/src/testing/testing.go:1934
runtime.goexit
/usr/local/go/src/runtime/asm_amd64.s:1693
Test: TestForwardTestSuite
--- FAIL: TestForwardTestSuite (0.29s)
```

### CI link

https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/tikv_pd/10181/pull-unit-test-next-gen-2/2018202086189043712

### Reason for failure (if possible)

### Anything else