Add test for issue #676

This concerns using DigraphAllSimpleCircuits with
digraphs with non-standard vertex labels.

tst/standard/attr.tst

@@ -948,6 +948,12 @@ gap> gr := Digraph([[3, 6, 7], [3, 6, 8], [1, 2, 3, 6, 7, 8],
 gap> Length(DigraphAllSimpleCircuits(gr));
 259
 
+#  Issue #676
+gap> D := Digraph([[], [3], []]);;
+gap> SetDigraphVertexLabels(D, ["one", "two", "three"]);
+gap> DigraphAllSimpleCircuits(D);
+[  ]
+
 #  DigraphLongestSimpleCircuit
 gap> gr := Digraph([]);;
 gap> DigraphLongestSimpleCircuit(gr);