This sample MVC4 aysnc should work, such as:

    public class HomeController : AsyncController
    {
        [AsyncTimeout(2500)]
        public async Task<ActionResult> Index()
        {
            WebClient wc = new WebClient();

            var result = await wc.DownloadStringTaskAsync("http://aspnet.codeplex.com/project/feeds/rss");

            ViewBag.Message = "test: " + result;

            return View();
        }
       

For unit test:

       change

         ViewResult result = controller.Index() as ViewResult; 

       to
            ViewResult result = controller.Index().GetAwaiter().GetResult() as ViewResult;