Make CompactionJobStatsTest.UniversalCompactionTest more robust

Summary:
CompactionJobStatsTest.UniversalCompactionTest assumes compaction
kicks in when the number of L0 files equals to the compaction trigger.
However, in some case, the compaction might not catch up the write
speed and thus compaction might not kick in until the number of L0 files
is GREATER than the compaction trigger.

This patch tries to fix this corner case by making the Put thread wait
for a potential compaction whenever it flushes.

Test Plan: ./compaction_job_stats_test

Reviewers: sdong, anthony, IslamAbdelRahman, igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D47589

db/compaction_job_stats_test.cc

@@ -1014,8 +1014,8 @@ TEST_P(CompactionJobStatsTest, UniversalCompactionTest) {
         &rnd, start_key, start_key + key_base - 1,
         kKeySize, kValueSize, key_interval,
         compression_ratio, 1);
-  }
     reinterpret_cast<DBImpl*>(db_)->TEST_WaitForCompact();
+  }
   ASSERT_EQ(stats_checker->NumberOfUnverifiedStats(), 0U);
 }