fix valgrind failure in backupable_db_test

Summary: fix memory leak in test code

Test Plan: ran test

Reviewers: rven, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D52617

utilities/backupable/backupable_db_test.cc

@@ -1321,6 +1321,8 @@ TEST_F(BackupableDBTest, Issue921Test) {
   backupable_options_->backup_dir += "/new_dir";
 
   ASSERT_OK(BackupEngine::Open(env_, *backupable_options_, &backup_engine));
+
+  delete backup_engine;
 }
 
 }  // anon namespace