message: Add basic A* test
  The current A* test doesn't test the heuristic usage, but simply
  validates that the pathfinding works.
diff: @@ -35,6 +35,7 @@ use petgraph::visit::{
  use petgraph::algo::{
  DfsSpace,
  dijkstra,
  + astar,
  };
  use petgraph::dot::{
  @@ -342,6 +343,32 @@ fn dijk() {
  assert_eq!(scores[&c], 9);
  }
  +#[test]
  +fn astar_test() {
  + let mut g = Graph::new();
  + let a = g.add_node("A");
  + let b = g.add_node("B");
  + let c = g.add_node("C");
  + let d = g.add_node("D");
  + let e = g.add_node("E");
  + let f = g.add_node("F");
  + g.add_edge(a, b, 7);
  + g.add_edge(c, a, 9);
  + g.add_edge(a, d, 14);
  + g.add_edge(b, c, 10);
  + g.add_edge(d, c, 2);
  + g.add_edge(d, e, 9);
  + g.add_edge(b, f, 15);
  + g.add_edge(c, f, 11);
  + g.add_edge(e, f, 6);
  +
  + let path = astar(&g, a, e, |e| *e.weight(), |_| 0);
  + assert_eq!(path, Some(vec![a, d, e]));
  +
  + let path = astar(&g, e, b, |e| *e.weight(), |_| 0);
  + assert_eq!(path, None);
  +}
  +
  #[cfg(feature = "generate")]
  #[test]
  fn test_generate_undirected() {
  