Make DBTest.CompressionStatsTest more deterministic

Summary:
DBTest.CompressionStatsTest on non_shm test where the storage device is slow

DBTest.CompressionStatsTest assumes that a flush happens to check the number of compressed blocks.
This is not always true if the Flush is slow, make the test more deterministic by forcing a flush before doing the check

Test Plan: Run the test locally

Reviewers: andrewkr, yiwu, lightmark, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61317

db/db_test.cc

@@ -4731,6 +4731,7 @@ TEST_F(DBTest, CompressionStatsTest) {
     // compressible string
     ASSERT_OK(Put(Key(i), RandomString(&rnd, 128) + std::string(128, 'a')));
   }
+  ASSERT_OK(Flush());
   ASSERT_GT(options.statistics->getTickerCount(NUMBER_BLOCK_COMPRESSED), 0);
 
   for (int i = 0; i < kNumKeysWritten; ++i) {
@@ -4750,6 +4751,7 @@ TEST_F(DBTest, CompressionStatsTest) {
     // compressible string
     ASSERT_OK(Put(Key(i), RandomString(&rnd, 128) + std::string(128, 'a')));
   }
+  ASSERT_OK(Flush());
   ASSERT_EQ(options.statistics->getTickerCount(NUMBER_BLOCK_COMPRESSED)
             - currentCompressions, 0);