support db_bench compact benchmark on bottommost files

Summary:
Without this option, running the compact benchmark on a DB containing only bottommost files simply returned immediately.
Closes https://github.com/facebook/rocksdb/pull/3138

Differential Revision: D6256660

Pulled By: ajkr

fbshipit-source-id: e3b64543acd503d821066f4200daa201d4fb3a9d

tools/db_bench_tool.cc

@@ -5183,7 +5183,9 @@ void VerifyDBFromDB(std::string& truth_db_name) {
 
   void Compact(ThreadState* thread) {
     DB* db = SelectDB(thread);
-    db->CompactRange(CompactRangeOptions(), nullptr, nullptr);
+    CompactRangeOptions cro;
+    cro.bottommost_level_compaction = BottommostLevelCompaction::kForce;
+    db->CompactRange(cro, nullptr, nullptr);
   }
 
   void CompactAll() {