Since the HTML result of `readable_author` is very predictable, short and static I'd probably directly assert the full HTML.
This would increase the readability a lot and you could drop the `assert_link_attributes` helper.
Example:
```python

    def test_linked_id_if_user_exists_with_no_fullname_anonymous_allowed(self):
        self.set_allow_anonymous_view_about(True)
        self.register_user('lara')
        self.assertEquals('<a href="http://nohost/plone/author/lara/">lara</a>', readable_author(object, 'lara'))
```

I think repeating the URL and asserting HTML in general is ok here since it is still quite short..