It is way too hard to fake a call to Controller.Url.IsLocalUrl, which means I failed as a tester that day.

CreateUrlHelperToFakeIsLocalUrl

public static UrlHelper CreateUrlHelperToFakeIsLocalUrl(string fakeRequestUrl)
{
    var httpContextBase = new Mock<HttpContextBase>();
    httpContextBase.Setup(x => x.Request.Url).Returns(new Uri(fakeRequestUrl));
    var requestContext = new RequestContext(httpContextBase.Object, new RouteData());

    return new UrlHelper(requestContext);
}

// controller.Url = CreateUrlHelperToFakeIsLocalUrl("http://aFakeHost/aFakePage");