Fix flakiness in FIFOCompaction test (github issue #573)

Summary:
The problem is that sometimes two memtables will be compacted together into a single file. In that case, our assertion

        ASSERT_EQ(NumTableFilesAtLevel(0), 5);

fails because same amount of data is in 4 files instead of 5. We should wait for flush so that we prevent two memtables merging into a single file.

Test Plan: `for i in `seq 20`; do mrtest FIFOCompactionTest; done` -- fails at least once before. fails zero times after.

Reviewers: rven

Reviewed By: rven

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D36939

db/db_test.cc

@@ -9613,6 +9613,7 @@ TEST_F(DBTest, FIFOCompactionTest) {
         ASSERT_OK(Put(ToString(i * 100 + j), RandomString(&rnd, 1024)));
       }
       // flush should happen here
+      ASSERT_OK(dbfull()->TEST_WaitForFlushMemTable());
     }
     if (iter == 0) {
       ASSERT_OK(dbfull()->TEST_WaitForCompact());