Don't use the [RequiresAuthentication] attribute from Rob's blog. It's meant for a very old pre-release version of MVC. Use the in-box [Authorize] attribute instead.
Since the [Authorize] attribute is written by the MVC team, you don't need to unit test its logic. However, if you want, you can verify that it's applied to your controllers or actions. Simply get the Type or MethodInfo you're interested in, then call its GetCustomAttributes() method to get instances of AuthorizeAttribute. You can inspect those instances for the values that you expect.
If you want, you can look at the source code of AuthorizeAttribute for information on writing your own filter. Additionally, you can look at the official unit test of this type, so if you do end up writing a filter you can use a similar method to write your own type's unit tests.