Switch to using freezegun for tests that depend on date.

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.

Create a factory module per app and move them under the new
oscar.test.factories module.

Before this change, the type of a bankcard would be clobbered by saving
and reloading.

Fixes #1486, #1487

TestCase.assertEquals (note the s at the end) has been deprecated.
The only failing tests remaining are CSV related, which is not
surprising.

This sets up failing test for #1128

Write a few more tests at the same time.