Rewrite ContentViewTestCase

* Drop an unnecessary `@ddt` decorator.
* Make the test code more straightforward.

Rather magically, the test now succeeds.

    $ nosetests tests/foreman/api/test_contentview.py:ContentViewTestCase
    .
    ----------------------------------------------------------------------
    Ran 1 test in 19.436s

    OK

This fixes a Jenkins failure.
I've mentioned it as a suture improvement because there are other methods that have the same behaviour and for consistency I think they all should be refactored. So we will need a new issue to track this.