functional/tester: handle "raft.ErrProposalDropped"
functional/tester: handle "raft.ErrProposalDropped"

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

li-ang/etcd,sebrandon1/etcd,mqliang/etcd,sinsharat/etcd,joshix/etcd,coreos/etcd,sinsharat/etcd,rnaveiras/etcd,mkumatag/etcd,li-ang/etcd,glevand/coreos--etcd,ajityagaty/etcd,gyuho/etcd,bdarnell/etcd,cockroachdb/etcd,bdarnell/etcd,dannysauer/etcd,etcd-io/etcd,gyuho/etcd,fanminshi/etcd,mkumatag/etcd,mkumatag/etcd,rnaveiras/etcd,ajityagaty/etcd,sebrandon1/etcd,etcd-io/etcd,vimalk78/etcd,joshix/etcd,vimalk78/etcd,mqliang/etcd,xiang90/etcd,xiang90/etcd,lorneli/etcd,fanminshi/etcd,lorneli/etcd,dannysauer/etcd,mqliang/etcd,rnaveiras/etcd,etcd-io/etcd,coreos/etcd,ajityagaty/etcd,glevand/coreos--etcd,cockroachdb/etcd