[tests] Add missing tests for SETNX.
[tests] Add missing tests for SETNX.

For some obscure reason Predis/Command/StringSetPreserve had no tests.

gopalindians/predis,RudyJessop/predis,moria/predis,zhangyancoder/predis,johnhelmuth/predis,nguyenthaihan/predis,lvbaosong/predis,oswaldderiemaecker/predis,WalterShe/predis,nrk/predis,vend/predis,quangnguyen90/predis,SwelenFrance/predis,CloudSide/predis,stokes3452/predis,dzung2t/predis,SecureCloud-biz/predis,gencer/predis,protomouse/predis,mrkeng/predis