I broke this in 6439a95. Use pytest to paramatrize this in a proper
way

This also removes the attr() marking of tests. This can be implemented
in pytest with marking the tests with a custom marker if required.

Django now throws a ValueError when an unsaved instance is assigned:
https://docs.djangoproject.com/en/1.8/releases/1.8/#assigning-unsaved-objects-to-relations-raises-an-error

This commit changes the offending lines to create objects in the
database instead. But I don't understand why I don't need to make
the same changes for e.g. the country factory.

But I don't really care, as long as the tests pass. I'm guessing it has
something to do with factory-boy and the get_or_create statement.

Unfortunately, this will make our test suite slower. But to make up for
it, Django 1.8 introduces the setUpTestCase method, which we can use
once Django 1.7 is removed.

As previously discussed here
https://groups.google.com/d/topic/django-oscar/Owmi3J1rwlk/discussion
I have started working on the introduction of Factoryboy library into
django-oscar, to make the generation of data for unit tests easier. At
the moment I have only refactored test/unit/address/models_tests.py, but
the plan is to refactor all the existing tests. All the tests, of
course, pass.

Squashed commit of the work done in #1192
Fixes #1192

Plus reorganise tests

This change adds a uniqueness constraint on user addresses to prevent
duplicates.  A few changes follow:

- The user address forms in account and checkout are modified to take a
  user as a constructor arg so uniqueness can be checked.

- The assignment of a user when creating a new address has been moved
  into the form.