Summary:
`CheckpointTest.CurrentFileModifiedWhileCheckpointing` could hang
because now create checkpoint triggers flush twice. The test should wait
both flush done.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7849
Test Plan: `gtest-parallel ./checkpoint_test --gtest_filter=CheckpointTest.CurrentFileModifiedWhileCheckpointing -r 100`
Reviewed By: ajkr
Differential Revision: D25860713
Pulled By: jay-zhuang
fbshipit-source-id: e1c2f23037dedc33e205519f4289a25e77816b41