Here is one of my tests (xUnit + Moq) just for similar case (using Url.RouteUrl in controller)
Hope this helps:
var routes = new RouteCollection();
MvcApplication.RegisterRoutes(routes);
var request = new Mock<HttpRequestBase>(MockBehavior.Strict);
request.SetupGet(x => x.ApplicationPath).Returns("https://stackoverflow.com/");
request.SetupGet(x => x.Url).Returns(new Uri("http://localhost/a", UriKind.Absolute));
request.SetupGet(x => x.ServerVariables).Returns(new System.Collections.Specialized.NameValueCollection());
var response = new Mock<HttpResponseBase>(MockBehavior.Strict);
response.Setup(x => x.ApplyAppPathModifier("/post1")).Returns("http://localhost/post1");
var context = new Mock<HttpContextBase>(MockBehavior.Strict);
context.SetupGet(x => x.Request).Returns(request.Object);
context.SetupGet(x => x.Response).Returns(response.Object);
var controller = new LinkbackController(dbF.Object);
controller.ControllerContext = new ControllerContext(context.Object, new RouteData(), controller);
controller.Url = new UrlHelper(new RequestContext(context.Object, new RouteData()), routes);