graph: add IsZombieEdge method

This is in preparation for the commit where we move across all the
funding tx validation so that we can test that we are correctly updating
the zombie index.

discovery/gossiper_test.go

@@ -113,6 +113,17 @@ func (r *mockGraphSource) MarkZombieEdge(scid uint64) error {
 	)
 }
 
+func (r *mockGraphSource) IsZombieEdge(chanID lnwire.ShortChannelID) (bool,
+	error) {
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	_, ok := r.zombies[chanID.ToUint64()]
+
+	return ok, nil
+}
+
 func (r *mockGraphSource) AddEdge(info *models.ChannelEdgeInfo,
 	_ ...batch.SchedulerOption) error {