Restrict running condition of UniversalCompactionTrivialMoveTest2

Summary: DBTestUniversalCompaction.UniversalCompactionTrivialMoveTest2 verifies non-trivial move is not triggered if we load data in sequential order. However, if there are multiple compaction threads, this conditon may not hold. Restrict the running condition to 1 compaction thread to make the test more robust.

Test Plan: Run the test and make sure at least it doesn't regress normally.

Reviewers: yhchiang, andrewkr, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D65277

db/db_universal_compaction_test.cc

@@ -1044,7 +1044,7 @@ TEST_P(DBTestUniversalCompaction, UniversalCompactionTrivialMoveTest2) {
   options.num_levels = 15;
   options.write_buffer_size = 100 << 10;  // 100KB
   options.level0_file_num_compaction_trigger = 8;
-  options.max_background_compactions = 4;
+  options.max_background_compactions = 1;
   options.target_file_size_base = 64 * 1024;
   DestroyAndReopen(options);
   CreateAndReopenWithCF({"pikachu"}, options);