Fix DBTest.UniversalCompactionTrigger failure caused by D17067

Summary: D17067 breaks DBTest.UniversalCompactionTrigger because of wrong location of the checking. Fix it.

Test Plan: Run the test and make sure it passes.

Reviewers: igor, haobo

Reviewed By: igor

CC: dhruba, ljin, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D17079