[RIP-10]optimized DefaultMessageStoreTest: invoke flush consume queue instead of sleeping few seconds

[wnow20]

What is the purpose of the change

optimized DefaultMessageStoreTest: invoke flush consume queue instead of sleeping few seconds

Brief changelog

1. call StoreTestUtil.flushConsumeQueue instead of Thread.sleep.
2. modify method names of test cases.
3. arrange private method that appending to the end of the file.
4. divide one testRecover into three smaller cases.

Verifying this change

All tests include DefaultMessageStoreTest run pass.

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[coveralls]

Coverage increased (+0.1%) to 49.057% when pulling d635f9f on wnow20:wnow20_store_fix_sleep_tricks into bbb723f on apache:develop.