raft: give correct offset in unstable test
raft: give correct offset in unstable test

`unstable.entries[i] has raft log position i+unstable.offset`

So, this fixes some test cases by giving them correct
offsets.

sinsharat/etcd,bdarnell/etcd,xiang90/etcd,whuwxl/etcd,sofuture/etcd,armstrongli/etcd,coreos/etcd,mickep76/etcd,purpleidea/etcd,insequent/etcd,armstrongli/etcd,luxas/etcd,sebrandon1/etcd,jlamillan/etcd,westhood/etcd,endocode/etcd,swingbach/etcd,ktateish/etcd,CrazyJvm/etcd,ajityagaty/etcd,sinsharat/etcd,cockroachdb/etcd,zaphod-concur/etcd,glevand/coreos--etcd,visheshnp/etcd,westhood/etcd,glycerine/etcd,ajityagaty/etcd,glevand/coreos--etcd,joshix/etcd,glycerine/etcd,luxas/etcd,jonboulle/etcd,fanminshi/etcd,StorReduce/etcd,ajityagaty/etcd,mkumatag/etcd,a-robinson/etcd,gyuho/etcd,cockroachdb/etcd,nekto0n/etcd,sebrandon1/etcd,jonboulle/etcd,lorneli/etcd,visheshnp/etcd,swingbach/etcd,purpleidea/etcd,mkumatag/etcd,rnaveiras/etcd,etcd-io/etcd,heyitsanthony/etcd,li-ang/etcd,handsomesun/etcd,sofuture/etcd,lumjjb/etcd,li-ang/etcd,rchicoli/etcd,endocode/etcd,whuwxl/etcd,fanminshi/etcd,YuleiXiao/etcd,wantonsolutions/dinv-etcd,StorReduce/etcd,dannysauer/etcd,ravigadde/etcd,mqliang/etcd,mqliang/etcd,dannysauer/etcd,yudai/etcd,bluebreezecf/etcd,akauppi/etcd,tguangch/etcd,bdarnell/etcd,insequent/etcd,hongchaodeng/etcd,ravigadde/etcd,a-robinson/etcd,linuxcer/etcd,OyTao/etcd_learning,mqliang/etcd,handsomesun/etcd,zaphod-concur/etcd,tguangch/etcd,jlamillan/etcd,YuleiXiao/etcd,rnaveiras/etcd,ktateish/etcd,plasticbox/etcd,rnaveiras/etcd,etcd-io/etcd,akauppi/etcd,wantonsolutions/dinv-etcd,bluebreezecf/etcd,CrazyJvm/etcd,linuxcer/etcd,joshix/etcd,qqsun8819/etcd,hongchaodeng/etcd,gyuho/etcd,yudai/etcd,qqsun8819/etcd,wantonsolutions/dinv-etcd,lumjjb/etcd,xiang90/etcd,etcd-io/etcd,coreos/etcd,vimalk78/etcd,vimalk78/etcd,plasticbox/etcd,mkumatag/etcd,lorneli/etcd,nekto0n/etcd,OyTao/etcd_learning,heyitsanthony/etcd,rchicoli/etcd,mickep76/etcd