backupable_db_test.cc: pass const string param by reference
backupable_db_test.cc: pass const string param by reference

Signed-off-by: Danny Al-Gaaf <473e39ac77cf392f0dd9bacde8409de76c14d006@bisect.de>

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