Fix flaky test: FileWatchServiceTest#watchTwoFiles_ModifyOne

[aaron-ai]

1. https://github.com/apache/rocketmq/runs/8007522493?check_suite_focus=true:
   
2. https://github.com/apache/rocketmq/runs/8006783346?check_suite_focus=true:
   

[aaron-ai]

Fixed by #4927