public void dumb_duck_cannot_quack()
 {
     Duck dumb = new DumbDuck();
     Assert.AreEqual("<< Silence >>", dumb.PerformQuack());
 }
 public void duck_is_a_dumb_duck()
 {
     Duck dumb = new DumbDuck();
     Assert.AreEqual("I'm a dumb duck.", dumb.Display());
 }