dag: test contains — heartwood

Test coverage for the Dag::contains method were not covered. Add tests for the method to cover an empty and filled Dag.

         );
         assert_eq!(order, vec!["R", "A3", "A2", "B3", "A1", "B2", "B1", "C1"]);
     }
     #[test]
     fn test_contains() {
         let mut dag = Dag::<u8, ()>::new();
         assert!(!dag.contains(&0));
         dag.node(0, ());
         dag.node(1, ());
         dag.dependency(0, 1);
         dag.node(2, ());
         dag.dependency(2, 1);
         dag.dependency(2, 0);
         dag.node(3, ());
         for i in 0..4 {
             assert!(dag.contains(&i));
         }
         assert!(!dag.contains(&4));
     }
 }