Added test for the login_required property

I have added a test for the login_required property of a Page object.

Unfortunately the login_required property doesn't provide the flexibility I need for an upcoming project. I think I will need something like "group_required" or "groups". If a logged-in user is member of a group in that groups property he is allowed to view the page.

There a two third-party modules providing a similar functionality but in my opinion this could/should be a core functionality because with a relative small change a lot of flexibility can be achieved.

If you agree I could look into the necessary changes and create another pull request.