Add two simple tests for DFS traversal

Author: LinjianMa

test/test_abstractgraph.jl

@@ -39,6 +39,11 @@ using NamedGraphs
   @test vertex_path(ng2, (1, 1, 2), (1, 1, 1)) == [(1, 1, 2), (1, 1), (1, 1, 1)]
   @test edge_path(ng2, (1, 1, 2), (1, 1, 1)) ==
     [net2((1, 1, 2), (1, 1)), net2((1, 1), (1, 1, 1))]
+  # Test DFS traversals
+  @test post_order_dfs_vertices(ng2, (1,)) ==
+    [(1, 1, 1), (1, 1, 2), (1, 1), (1, 2, 1), (1, 2, 2), (1, 2), (1,)]
+  @test pre_order_dfs_vertices(ng2, (1,)) ==
+    [(1,), (1, 1), (1, 1, 1), (1, 1, 2), (1, 2), (1, 2, 1), (1, 2, 2)]
 
   # directed trees
   dg1 = dfs_tree(g1, 5)