template tag test refactor

Test the tag in template use, not the Node. Removing the implementation details is the same change we made to ArmLayout in armstrong/armstrong.core.arm_layout#12. Add a couple new scenarios as well.

I did this so we could refactor the template tag into a `simple_tag`. Piece of cake. Except for Django 1.3. Currently, and I'd like to maintain this, the tag only accepts `kwargs`. I don't want to change the behavior and I like the explicit meaning when using kwargs. Call me nostalgic, but the code already exists and we've maintained compatibility so far. I'd like to keep Django 1.3 support.

In Django 1.4 [simple_tag](https://docs.djangoproject.com/en/1.4/howto/custom-template-tags/#simple-tags) can take arbitrary kwargs.
> New in Django 1.4: simple_tag functions may accept any number of positional or keyword arguments.