Summary: DBTest.MergeTestTime is a test verifying timing counters. Depending on real time may cause non-determinstic results. Change to fake time to be determinsitic.

Test Plan: Run the test and make sure it passes

Reviewers: yhchiang, anthony, rven, kradhakrishnan, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D50883