Summary:
Currently, this test case tries to infer whether
`VersionStorageInfo::UpdateAccumulatedStats` was called during open by
checking the number of files opened against an arbitrary threshold (10).
This makes the test brittle and results in sporadic failures. The patch
changes the test case to use sync points to directly test whether
`UpdateAccumulatedStats` was called.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6306

Test Plan: `make check`

Differential Revision: D19439544

Pulled By: ltamasi

fbshipit-source-id: ceb7adf578222636a0f51740872d0278cd1a914f