integration: clean up resources in error paths of TestV3WatchFromCurrentRevision
integration: clean up resources in error paths of TestV3WatchFromCurrentRevision

Current error paths of TestV3WatchFromCurrentRevision don't clean the
used resources including goroutines. Because go's tests are executed
continuously in a single process, the leaked goroutines makes error
logs bloated like the below case:
https://jenkins-etcd-public.prod.coreos.systems/job/etcd-coverage/2143/

This commit lets the error paths clean the resources.

sinsharat/etcd,fanminshi/etcd,etcd-io/etcd,ajityagaty/etcd,vimalk78/etcd,mkumatag/etcd,akauppi/etcd,mqliang/etcd,visheshnp/etcd,whuwxl/etcd,a-robinson/etcd,bluebreezecf/etcd,etcd-io/etcd,sebrandon1/etcd,YuleiXiao/etcd,gyuho/etcd,mkumatag/etcd,a-robinson/etcd,akauppi/etcd,mickep76/etcd,mickep76/etcd,bdarnell/etcd,glevand/coreos--etcd,heyitsanthony/etcd,visheshnp/etcd,lorneli/etcd,cockroachdb/etcd,dannysauer/etcd,dannysauer/etcd,joshix/etcd,mqliang/etcd,heyitsanthony/etcd,joshix/etcd,rnaveiras/etcd,cockroachdb/etcd,mqliang/etcd,sebrandon1/etcd,xiang90/etcd,xiang90/etcd,mkumatag/etcd,YuleiXiao/etcd,ajityagaty/etcd,sinsharat/etcd,glevand/coreos--etcd,ajityagaty/etcd,bdarnell/etcd,whuwxl/etcd,bluebreezecf/etcd,lorneli/etcd,coreos/etcd,gyuho/etcd,coreos/etcd,li-ang/etcd,vimalk78/etcd,fanminshi/etcd,rnaveiras/etcd,li-ang/etcd,etcd-io/etcd,rnaveiras/etcd