Improve listener_test by ensuring flushes are completed before assert.

Summary: Improve listener_test by ensuring flushes are completed before assert.

Test Plan: listener_test

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D29319

db/listener_test.cc

@@ -135,10 +135,12 @@ class EventListenerTest {
   }
 
   Status Flush(int cf = 0) {
+    FlushOptions opt = FlushOptions();
+    opt.wait = true;
     if (cf == 0) {
-      return db_->Flush(FlushOptions());
+      return db_->Flush(opt);
     } else {
-      return db_->Flush(FlushOptions(), handles_[cf]);
+      return db_->Flush(opt, handles_[cf]);
     }
   }