Testing controller Action that uses User.Identity.Name

You can use this code

public SomeController CreateControllerForUser(string userName) 
{
    var mock = new Mock<ControllerContext>();
    mock.SetupGet(p => p.HttpContext.User.Identity.Name).Returns(userName);
    mock.SetupGet(p => p.HttpContext.Request.IsAuthenticated).Returns(true);

    var controller = new SomeController();
    controller.ControllerContext = mock.Object;

    return controller;
}

It uses Moq mocking framework, but sure you can use anything you like.