Clean-up WAL directory before running db_test
Clean-up WAL directory before running db_test

Summary: DBTest doesn't clean up wal directory. It might cause failure after a failure test run. Fix it.

Test Plan:
Run unit tests
Try open DB with non-empty db_path/wal.

Reviewers: rven, yhchiang, kradhakrishnan, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D35559

facebook/rocksdb,tsheasha/rocksdb,flabby/rocksdb,amyvmiwei/rocksdb,ylong/rocksdb,virtdb/rocksdb,bbiao/rocksdb,luckywhu/rocksdb,rDSN-Projects/rocksdb.replicated,alihalabyah/rocksdb,dkorolev/rocksdb,zhangpng/rocksdb,zhangpng/rocksdb,siddhartharay007/rocksdb,RyanTech/rocksdb,sorphi/rocksdb,vmx/rocksdb,rDSN-Projects/rocksdb.replicated,Andymic/rocksdb,kaschaeffer/rocksdb,norton/rocksdb,vashstorm/rocksdb,rDSN-Projects/rocksdb.replicated,IMCG/RcoksDB,mbarbon/rocksdb,facebook/rocksdb,bbiao/rocksdb,alihalabyah/rocksdb,rDSN-Projects/rocksdb.replicated,vmx/rocksdb,amyvmiwei/rocksdb,temicai/rocksdb,geraldoandradee/rocksdb,caijieming-baidu/rocksdb,luckywhu/rocksdb,bbiao/rocksdb,luckywhu/rocksdb,bbiao/rocksdb,kaschaeffer/rocksdb,mbarbon/rocksdb,tsheasha/rocksdb,JohnPJenkins/rocksdb,RyanTech/rocksdb,NickCis/rocksdb,jalexanderqed/rocksdb,jalexanderqed/rocksdb,flabby/rocksdb,anagav/rocksdb,caijieming-baidu/rocksdb,temicai/rocksdb,vmx/rocksdb,luckywhu/rocksdb,biddyweb/rocksdb,sorphi/rocksdb,kaschaeffer/rocksdb,amyvmiwei/rocksdb,lgscofield/rocksdb,luckywhu/rocksdb,bbiao/rocksdb,virtdb/rocksdb,dkorolev/rocksdb,OverlordQ/rocksdb,facebook/rocksdb,RyanTech/rocksdb,wenduo/rocksdb,anagav/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,JoeWoo/rocksdb,JackLian/rocksdb,ryneli/rocksdb,siddhartharay007/rocksdb,tizzybec/rocksdb,sorphi/rocksdb,JoeWoo/rocksdb,tsheasha/rocksdb,wat-ze-hex/rocksdb,alihalabyah/rocksdb,hobinyoon/rocksdb,facebook/rocksdb,fengshao0907/rocksdb,siddhartharay007/rocksdb,lgscofield/rocksdb,dkorolev/rocksdb,vmx/rocksdb,wenduo/rocksdb,temicai/rocksdb,RyanTech/rocksdb,vmx/rocksdb,tsheasha/rocksdb,vashstorm/rocksdb,jalexanderqed/rocksdb,biddyweb/rocksdb,skunkwerks/rocksdb,IMCG/RcoksDB,wat-ze-hex/rocksdb,geraldoandradee/rocksdb,hobinyoon/rocksdb,ylong/rocksdb,makelivedotnet/rocksdb,amyvmiwei/rocksdb,ryneli/rocksdb,ylong/rocksdb,lgscofield/rocksdb,anagav/rocksdb,tizzybec/rocksdb,wskplho/rocksdb,bbiao/rocksdb,wskplho/rocksdb,hobinyoon/rocksdb,norton/rocksdb,siddhartharay007/rocksdb,RyanTech/rocksdb,JoeWoo/rocksdb,Andymic/rocksdb,ylong/rocksdb,alihalabyah/rocksdb,OverlordQ/rocksdb,alihalabyah/rocksdb,RyanTech/rocksdb,fengshao0907/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,Vaisman/rocksdb,sorphi/rocksdb,wskplho/rocksdb,lgscofield/rocksdb,mbarbon/rocksdb,skunkwerks/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,amyvmiwei/rocksdb,flabby/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,luckywhu/rocksdb,lgscofield/rocksdb,wat-ze-hex/rocksdb,vashstorm/rocksdb,flabby/rocksdb,virtdb/rocksdb,zhangpng/rocksdb,norton/rocksdb,OverlordQ/rocksdb,makelivedotnet/rocksdb,temicai/rocksdb,ryneli/rocksdb,fengshao0907/rocksdb,JackLian/rocksdb,sorphi/rocksdb,wlqGit/rocksdb,geraldoandradee/rocksdb,mbarbon/rocksdb,SunguckLee/RocksDB,rDSN-Projects/rocksdb.replicated,norton/rocksdb,flabby/rocksdb,tizzybec/rocksdb,alihalabyah/rocksdb,Applied-Duality/rocksdb,SunguckLee/RocksDB,mbarbon/rocksdb,caijieming-baidu/rocksdb,SunguckLee/RocksDB,jalexanderqed/rocksdb,wlqGit/rocksdb,JoeWoo/rocksdb,facebook/rocksdb,zhangpng/rocksdb,NickCis/rocksdb,vashstorm/rocksdb,biddyweb/rocksdb,makelivedotnet/rocksdb,geraldoandradee/rocksdb,facebook/rocksdb,geraldoandradee/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,JohnPJenkins/rocksdb,siddhartharay007/rocksdb,JohnPJenkins/rocksdb,Applied-Duality/rocksdb,vmx/rocksdb,JohnPJenkins/rocksdb,norton/rocksdb,wlqGit/rocksdb,tsheasha/rocksdb,vmx/rocksdb,fengshao0907/rocksdb,kaschaeffer/rocksdb,skunkwerks/rocksdb,skunkwerks/rocksdb,wenduo/rocksdb,ylong/rocksdb,JackLian/rocksdb,SunguckLee/RocksDB,siddhartharay007/rocksdb,lgscofield/rocksdb,JohnPJenkins/rocksdb,makelivedotnet/rocksdb,alihalabyah/rocksdb,wenduo/rocksdb,OverlordQ/rocksdb,wskplho/rocksdb,wenduo/rocksdb,tizzybec/rocksdb,dkorolev/rocksdb,tizzybec/rocksdb,wlqGit/rocksdb,temicai/rocksdb,bbiao/rocksdb,JoeWoo/rocksdb,mbarbon/rocksdb,wat-ze-hex/rocksdb,norton/rocksdb,virtdb/rocksdb,Applied-Duality/rocksdb,OverlordQ/rocksdb,geraldoandradee/rocksdb,wlqGit/rocksdb,facebook/rocksdb,RyanTech/rocksdb,amyvmiwei/rocksdb,sorphi/rocksdb,Andymic/rocksdb,rDSN-Projects/rocksdb.replicated,flabby/rocksdb,zhangpng/rocksdb,wlqGit/rocksdb,NickCis/rocksdb,ryneli/rocksdb,NickCis/rocksdb,anagav/rocksdb,zhangpng/rocksdb,Vaisman/rocksdb,rDSN-Projects/rocksdb.replicated,dkorolev/rocksdb,ryneli/rocksdb,fengshao0907/rocksdb,JohnPJenkins/rocksdb,kaschaeffer/rocksdb,NickCis/rocksdb,caijieming-baidu/rocksdb,Vaisman/rocksdb,amyvmiwei/rocksdb,Andymic/rocksdb,lgscofield/rocksdb,anagav/rocksdb,vashstorm/rocksdb,JohnPJenkins/rocksdb,sorphi/rocksdb,kaschaeffer/rocksdb,IMCG/RcoksDB,caijieming-baidu/rocksdb,biddyweb/rocksdb,fengshao0907/rocksdb,kaschaeffer/rocksdb,JoeWoo/rocksdb,tizzybec/rocksdb,IMCG/RcoksDB,ryneli/rocksdb,Applied-Duality/rocksdb,tizzybec/rocksdb,caijieming-baidu/rocksdb,dkorolev/rocksdb,JoeWoo/rocksdb,facebook/rocksdb,JackLian/rocksdb,NickCis/rocksdb,JackLian/rocksdb,bbiao/rocksdb,jalexanderqed/rocksdb,anagav/rocksdb,Applied-Duality/rocksdb,vmx/rocksdb,biddyweb/rocksdb,fengshao0907/rocksdb,makelivedotnet/rocksdb,g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