Most real-world instances will use postgresql instead of sqlite so it's
better to use postgres for the tests. This also allows us to easily add
tests for concurrency issues which is not supported by sqlite

As in `test_model.py` -> `test_models.py`

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.

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

Validation is based on wikipedia list of postal codes [1]. Formats were
converted into regexes for countries.

1: http://en.wikipedia.org/wiki/List_of_postal_codes

Fixes #753

Also I moved the generate_hash fn up to the Address superclass as it's
possible that it might be used in other Address classes.

Also split them into unit and functional tests.

to the admin and model set-up.

Completed the models for the order package too.  Added admin files