Unit testing controllers using UserManager.FindById - asp.net-mvc

Question

I am new to unit testing .net applications and am having difficulty with what I would imagine to be a very simple case.
// GET: Entities
public ViewResult Index()
{
_User = UserManager.FindById(User.Identity.GetUserId());
return View(entityRepository.GetEntities(_User.entityId));
}
I want to test that the correct view is outputted but can't get past the user line. In other languages I would simply mock UserManager.FindById to always return some predefined object, but I can't get it to work.
Have been trying to follow the approach given here example mocking the IUserStore but can't get it to work with my example. To mock FindByNameAsync they have used store.As
Any advice would be gratefully received.
My attempt was to follow a similar method to that in link above. Obviously IUserPasswordStore is the wrong interface, but I am not sure how to find the correct one.
var store = new Mock<IUserStore<ApplicationUser>>(MockBehavior.Strict);
store.As<IUserPasswordStore<ApplicationUser>>()
.Setup(x => x.FindById(It.IsAny<string>()))
.Returns(ApplicationUser)null);
EntitiesController controller = new EntitiesController();
var result = controller.Index() as ViewResult;
Assert.AreEqual("Index", result.ViewName);

Answer 1

So, thanks to the guidance from #Nkosi and #haim770 I think I have an answer. It still seems overly complex though, so would be interested to hear if you know how to simplify.
It started with needing to write more testable code, I installed Unity and started to inject dependencies allowing me to mock them out.
The solution that passes is:
Mock<IPrincipal> mockPrincipal;
string username = "test#test.com";
[TestInitialize]
public void TestInitialize()
{
//Arrange
var identity = new GenericIdentity(username, "");
var nameIdentifierClaim = new Claim(ClaimTypes.NameIdentifier, username);
identity.AddClaim(nameIdentifierClaim);
mockPrincipal = new Mock<IPrincipal>();
mockPrincipal.Setup(x => x.Identity).Returns(identity);
mockPrincipal.Setup(x => x.IsInRole(It.IsAny<string>())).Returns(true);
}
[TestMethod]
public void EntitiesIndexDisplaysTheDefaultView()
{
var context = new Mock<HttpContextBase>();
var principal = mockPrincipal.Object;
context.Setup(x => x.User).Returns(principal);
var userManagerMock = new Mock<IUserStore<ApplicationUser>>(MockBehavior.Strict);
userManagerMock.As<IUserPasswordStore<ApplicationUser>>()
.Setup(x => x.FindByIdAsync(It.IsAny<string>()))
.ReturnsAsync(new ApplicationUser() { entityId = "id" });
var entitiesRepositoryMock = new Mock<IEntityRepository>();
entitiesRepositoryMock
.Setup(x => x.GetEntities(It.IsAny<string>()))
.Returns((IEnumerable<Entity>)new List<Entity>());
EntitiesController controller = new EntitiesController(new UserManager<ApplicationUser>(userManagerMock.Object),
entitiesRepositoryMock.Object);
controller.ControllerContext = new ControllerContext(context.Object, new RouteData(), controller);
var result = controller.Index() as ViewResult;
Assert.AreEqual("", result.ViewName);
}