write_callback_test: clean test directory before running tests

Summary: write_callback_test fails if previous run didn't finish cleanly. Clean the DB before runing the test.

Test Plan: Run the test that see it doesn't fail any more.

Reviewers: andrewkr, yhchiang, yiwu, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: kradhakrishnan, leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56859

db/write_callback_test.cc

@@ -123,6 +123,7 @@ TEST_F(WriteCallbackTest, WriteWithCallbackTest) {
           DB* db;
           DBImpl* db_impl;
 
+          DestroyDB(dbname, options);
           ASSERT_OK(DB::Open(options, dbname, &db));
 
           db_impl = dynamic_cast<DBImpl*>(db);
@@ -282,6 +283,8 @@ TEST_F(WriteCallbackTest, WriteCallBackTest) {
   DB* db;
   DBImpl* db_impl;
 
+  DestroyDB(dbname, options);
+
   options.create_if_missing = true;
   Status s = DB::Open(options, dbname, &db);
   ASSERT_OK(s);