clientv3/balancer: use new mock server in tests
clientv3/balancer: use new mock server in tests

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

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