cdc,tests: unit test TestRecvLargeMessageSize is flaky

[pingyu]

Bug Report

1. Describe the bug

Unit test TestRecvLargeMessageSize is flaky:

----------------------------------------------------------------------
PANIC: client_test.go:431: clientSuite.TestRecvLargeMessageSize

... Panic: runtime error: invalid memory address or nil pointer dereference (PC=0x473D8F)

/usr/local/go/src/runtime/panic.go:920
  in gopanic
/usr/local/go/src/runtime/panic.go:261
  in panicmem
/usr/local/go/src/runtime/signal_unix.go:861
  in sigpanic
client_test.go:511
  in clientSuite.TestRecvLargeMessageSize
/usr/local/go/src/reflect/value.go:380
  in Value.Call
/go/pkg/mod/github.com/pingcap/check@v0.0.0-20211026125417-57bd13f7b5f0/check.go:850
  in suiteRunner.forkTest.func1
/go/pkg/mod/github.com/pingcap/check@v0.0.0-20211026125417-57bd13f7b5f0/check.go:739
  in suiteRunner.forkCall.func1
/usr/local/go/src/runtime/asm_amd64.s:1650
  in goexit

[2024/01/24 12:37:48.979 +08:00] [ERROR] [client_test.go:3323] ["mock server error"] [error="rpc error: code = Canceled desc = context canceled"] [stack="github.com/tikv/migration/cdc/cdc/kv.(*clientSuite).TestRegionWorkerExitWhenIsIdle.func1\n\t/home/jenkins/agent/workspace/tikv/migration/pull_unit_test/migration/cdc/cdc/kv/client_test.go:3323\ngithub.com/tikv/migration/cdc/cdc/kv.(*mockChangeDataService).EventFeed.func1\n\t/home/jenkins/agent/workspace/tikv/migration/pull_unit_test/migration/cdc/cdc/kv/client_test.go:252"]
[2024/01/24 12:38:12.904 +08:00] [ERROR] [client_test.go:1507] ["mock server error"] [error="rpc error: code = Canceled desc = context canceled"] [stack="github.com/tikv/migration/cdc/cdc/kv.(*clientSuite).TestStreamRecvWithErrorAndResolvedGoBack.func1\n\t/home/jenkins/agent/workspace/tikv/migration/pull_unit_test/migration/cdc/cdc/kv/client_test.go:1507\ngithub.com/tikv/migration/cdc/cdc/kv.(*mockChangeDataService).EventFeed.func1\n\t/home/jenkins/agent/workspace/tikv/migration/pull_unit_test/migration/cdc/cdc/kv/client_test.go:252"]
[2024/01/24 12:38:13.030 +08:00] [ERROR] [client_test.go:1507] ["mock server error"] [error="rpc error: code = Canceled desc = context canceled"] [stack="github.com/tikv/migration/cdc/cdc/kv.(*clientSuite).TestStreamRecvWithErrorAndResolvedGoBack.func1\n\t/home/jenkins/agent/workspace/tikv/migration/pull_unit_test/migration/cdc/cdc/kv/client_test.go:1507\ngithub.com/tikv/migration/cdc/cdc/kv.(*mockChangeDataService).EventFeed.func1\n\t/home/jenkins/agent/workspace/tikv/migration/pull_unit_test/migration/cdc/cdc/kv/client_test.go:252"]
[2024/01/24 12:38:20.272 +08:00] [ERROR] [client_test.go:1318] ["mock server error"] [error="rpc error: code = Canceled desc = context canceled"] [stack="github.com/tikv/migration/cdc/cdc/kv.(*clientSuite).TestStreamSendWithError.func4\n\t/home/jenkins/agent/workspace/tikv/migration/pull_unit_test/migration/cdc/cdc/kv/client_test.go:1318\ngithub.com/tikv/migration/cdc/cdc/kv.(*mockChangeDataService).EventFeed.func1\n\t/home/jenkins/agent/workspace/tikv/migration/pull_unit_test/migration/cdc/cdc/kv/client_test.go:252"]

OOPS: 48 passed, 1 skipped, 1 PANICKED

DONE 231 tests, 1 failure in 703.043s

See https://do.pingcap.net/jenkins/blue/organizations/jenkins/tikv%2Fmigration%2Fpull_unit_test/detail/pull_unit_test/33/pipeline/.

2. Minimal reproduce step (Required)

Run unit tests in CI.

3. What did you see instead (Required)

Test failed.

4. What did you expect to see? (Required)

Test succeed.

5. What is your migration tool and TiKV version? (Required)

• TiKV:
• TiKV CDC: [to #381] Add more integration tests for Kafka sink #387, e54141f
• TiKV BR:
• TiKV Online Bulk Load: