Asynchronous testing with NUnit?

I have a collection subclass who's items remove themselves after some duration:

 public class AttrModifierCollection : System.Collections.ObjectModel.Collection<AttrModifier> {
     protected override void InsertItem(int index, AttrModifier mod) {
         base.InsertItem(index, mod);
         if (mod.duration > 0) {
             Timer timer = new Timer(mod.duration);
             timer.Elapsed += delegate { Remove(mod); };
             timer.Start();
         }
     }
 }

I'm trying to write out a test for it but am having a very difficult time.

Here's the test as it currently is:

     [Test]
     public async void AttrModifier_Temporary()
     {
         mod = new AttrModifier("test attribute", 20f, 1f);
         attribute.modifiers.Add(mod);
         Assert.AreEqual(attribute.finalValue, 30f);

         Assert.That(attribute.finalValue, IsEqualTo(attrBaseValue).After(2));
     }

However, I keep receiving this error:

 Unexpected symbol `void' in class, struct, or interface member declaration

If I try removing the void keyword, so that the method declaration is just:

 public async AttrModifier_Temporary()

I instead get this error:

 CS0246: The type or namespace name `async' could not be found. Are you missing a using directive or an assembly reference?

I can't figure out what's wrong. I'm using Unity 4.3.4f1 on OSX 10.9.3.