Add beforeOpenSnapshotBackend failpoint

Signed-off-by: Marek Siarkowicz <siarkowicz@google.com>

server/etcdserver/server.go

@@ -1001,6 +1001,7 @@ func (s *EtcdServer) applySnapshot(ep *etcdProgress, toApply *toApply) {
 	// wait for raftNode to persist snapshot onto the disk
 	<-toApply.notifyc
 
+	// gofail: var beforeOpenSnapshotBackend struct{}
 	newbe, err := serverstorage.OpenSnapshotBackend(s.Cfg, s.snapshotter, toApply.snapshot, s.beHooks)
 	if err != nil {
 		lg.Panic("failed to open snapshot backend", zap.Error(err))