Fix url argument usage in NavigationTestMixin.
Fix url argument usage in NavigationTestMixin.

The not_at_page() method should prepend the live_server_url to the
url test so that the caller doesn't need to do this every time.  This
method uses urljoin for this purpose.  The url_should/not_contain()
method used url as the argument name, but it's really a text substring.
The name text is used to make it clearer.

nimbis/django-selenium-testcase,nimbis/django-selenium-testcase