[ROCKETMQ-321] Fix deleting files in the middle

[dongeforever]

What is the purpose of the change

Avoid deleting files in the middle.

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[coveralls]

Coverage increased (+0.05%) to 39.184% when pulling d1ff115 on dongeforever:ROCKETMQ-321 into 4926187 on apache:develop.

[zhouxinyu]

LGTM, but no need to return this file to test code, you could structure the File object directly.

[shroman]

+1