Relax FlushSchedule test

Summary: The test makes sure that we don't call flush too often. For that, it's ok to check if we have less than 10 table files. Otherwise, the test is flaky because it's hard to estimate number of entries in the memtable before it gets flushed (any ideas?)

Test Plan: Still works, but hopefully less flaky.

Reviewers: ljin, sdong, yhchiang

Reviewed by: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23241