Fix a valgrind warning

Summary:
A latest valgrind test found a recently added unit test has memory leak,
which is because DB is not closed at the end of the test.

Test Plan: re-run the valgrind locally and make sure there's no memory leakage any more.

Reviewers: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13725

db/deletefile_test.cc

@@ -218,6 +218,7 @@ TEST(DeleteFileTest, DeleteLogFiles) {
           archived_log->PathName().c_str());
   ASSERT_OK(db_->DeleteFile(archived_log->PathName()));
   ASSERT_TRUE(!env_->FileExists(dbname_ + "/" + archived_log->PathName()));
+  CloseDB();
 }
 
 } //namespace rocksdb