TestResignAPIPrimaryForward is unstable

## Flaky Test

### Which jobs are failing
```
2023-08-29T02:24:03.1901281Z panic: test timed out after 15m0s
2023-08-29T02:24:03.1901409Z running tests:
2023-08-29T02:24:03.1901628Z TestAPIServerForwardTestSuite (10m30s)
2023-08-29T02:24:03.1901947Z TestAPIServerForwardTestSuite/TestResignAPIPrimaryForward (8m21s)
2023-08-29T02:24:03.1902048Z
2023-08-29T02:24:03.1902195Z goroutine 2571455 [running]:
2023-08-29T02:24:03.1902412Z testing.(*M).startAlarm.func1()
2023-08-29T02:24:03.1902691Z /opt/hostedtoolcache/go/1.21.0/x64/src/testing/testing.go:2259 +0x1fc
2023-08-29T02:24:03.1902827Z created by time.goFunc
2023-08-29T02:24:03.1903074Z /opt/hostedtoolcache/go/1.21.0/x64/src/time/sleep.go:176 +0x45
2023-08-29T02:24:03.1903084Z
2023-08-29T02:24:03.1903251Z goroutine 1 [chan receive, 10 minutes]:
2023-08-29T02:24:03.1903448Z testing.(*T).Run(0xc0001861a0, {0x42f441f, 0x1d}, 0x4daf520)
2023-08-29T02:24:03.1903696Z /opt/hostedtoolcache/go/1.21.0/x64/src/testing/testing.go:1649 +0x856
2023-08-29T02:24:03.1903856Z testing.runTests.func1(0x0?)
2023-08-29T02:24:03.1904130Z /opt/hostedtoolcache/go/1.21.0/x64/src/testing/testing.go:2054 +0x85
2023-08-29T02:24:03.1904302Z testing.tRunner(0xc0001861a0, 0xc000665a30)
2023-08-29T02:24:03.1904566Z /opt/hostedtoolcache/go/1.21.0/x64/src/testing/testing.go:1595 +0x239
2023-08-29T02:24:03.1904821Z testing.runTests(0xc0002ed400?, {0x65a6b00, 0xc, 0xc}, {0x105c733?, 0x4?, 0x6676d60?})
2023-08-29T02:24:03.1905086Z /opt/hostedtoolcache/go/1.21.0/x64/src/testing/testing.go:2052 +0x897
2023-08-29T02:24:03.1905226Z testing.(*M).Run(0xc0002ed400)
2023-08-29T02:24:03.1905479Z /opt/hostedtoolcache/go/1.21.0/x64/src/testing/testing.go:1925 +0xb58
2023-08-29T02:24:03.1905744Z go.uber.org/goleak.VerifyTestMain({0x51e3a80, 0xc0002ed400}, {0x66787e0, 0x8, 0x8})
2023-08-29T02:24:03.1906054Z /home/runner/go/pkg/mod/go.uber.org/goleak@v1.1.12/testmain.go:53 +0x58
2023-08-29T02:24:03.1906279Z github.com/tikv/pd/tests/integrations/mcs/tso.TestMain(...)
2023-08-29T02:24:03.1906549Z /home/runner/work/pd/pd/tests/integrations/mcs/tso/server_test.go:51
2023-08-29T02:24:03.1906664Z main.main()
2023-08-29T02:24:03.1906826Z _testmain.go:111 +0x359
2023-08-29T02:24:03.1906839Z
2023-08-29T02:24:03.1906984Z goroutine 41 [chan receive]:
2023-08-29T02:24:03.1907228Z go.etcd.io/etcd/pkg/logutil.(*MergeLogger).outputLoop(0xc000013bd8)
2023-08-29T02:24:03.1907771Z /home/runner/go/pkg/mod/go.etcd.io/etcd@v0.5.0-alpha.5.0.20220915004622-85b640cee793/pkg/logutil/merge_logger.go:173 +0xa7
2023-08-29T02:24:03.1908037Z created by go.etcd.io/etcd/pkg/logutil.NewMergeLogger in goroutine 1
2023-08-29T02:24:03.1908567Z /home/runner/go/pkg/mod/go.etcd.io/etcd@v0.5.0-alpha.5.0.20220915004622-85b640cee793/pkg/logutil/merge_logger.go:91 +0x167
2023-08-29T02:24:03.1908576Z
2023-08-29T02:24:03.1908710Z goroutine 1018 [select]:
```

### CI link
https://github.com/tikv/pd/actions/runs/6006223367/job/16292464207?pr=6996

### Reason for failure (if possible)

### Anything else