Test widget can't be shown for unpublished story

Addresses #741
Add unit tests for list widget views

In order to avoid repeated code, create a mixin class with
common test initialization and test methods.

The logical home for this mixin class is
``storybase_story.tests.base``, so ``storybase_story/tests.py``
had to be moved to ``storybase_story/tests/__init__.py``

Addresses #741