Make DBTestUniversalCompaction.IncreaseUniversalCompactionNumLevels more robust

Summary:
Based on thread scheduling, DBTestUniversalCompaction.IncreaseUniversalCompactionNumLevels can fail to flush enough files to trigger expected compactions. Fix it by waiting for flush after inserting each key.
There are failrue reported:

db/db_universal_compaction_test.cc:1134: Failure
Expected: (NumTableFilesAtLevel(options.num_levels - 1, 1)) > (0), actual: 0 vs 0

but I can't repro it. Try to fix the bug and see whether it goes away.

Test Plan: Run the test multiple time.

Reviewers: IslamAbdelRahman, anthony, andrewkr, kradhakrishnan, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D54747

db/db_universal_compaction_test.cc

@@ -1103,16 +1103,11 @@ TEST_P(DBTestUniversalCompaction, IncreaseUniversalCompactionNumLevels) {
   for (int i = 0; i <= max_key1; i++) {
     // each value is 10K
     ASSERT_OK(Put(1, Key(i), RandomString(&rnd, 10000)));
+    dbfull()->TEST_WaitForFlushMemTable(handles_[1]);
   }
   ASSERT_OK(Flush(1));
   dbfull()->TEST_WaitForCompact();
 
-  int non_level0_num_files = 0;
-  for (int i = 1; i < options.num_levels; i++) {
-    non_level0_num_files += NumTableFilesAtLevel(i, 1);
-  }
-  ASSERT_EQ(non_level0_num_files, 0);
-
   // Stage 2: reopen with universal compaction, num_levels=4
   options.compaction_style = kCompactionStyleUniversal;
   options.num_levels = 4;
@@ -1125,6 +1120,7 @@ TEST_P(DBTestUniversalCompaction, IncreaseUniversalCompactionNumLevels) {
   for (int i = max_key1 + 1; i <= max_key2; i++) {
     // each value is 10K
     ASSERT_OK(Put(1, Key(i), RandomString(&rnd, 10000)));
+    dbfull()->TEST_WaitForFlushMemTable(handles_[1]);
   }
   ASSERT_OK(Flush(1));
   dbfull()->TEST_WaitForCompact();
@@ -1155,6 +1151,7 @@ TEST_P(DBTestUniversalCompaction, IncreaseUniversalCompactionNumLevels) {
   for (int i = max_key2 + 1; i <= max_key3; i++) {
     // each value is 10K
     ASSERT_OK(Put(1, Key(i), RandomString(&rnd, 10000)));
+    dbfull()->TEST_WaitForFlushMemTable(handles_[1]);
   }
   ASSERT_OK(Flush(1));
   dbfull()->TEST_WaitForCompact();