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

Before this change, it was conflating WebTest functionality with the
Django test client. This change removes the Django test client
functionality (namely, the login function which was being automatically
called).

A shed-load of tests are adjusted to work with this change.

This almost completes the work for #826 - only a couple more references
to the Django client are left in the test codebase.

ClientTestCase is deprecated.

Fixes #826
Fixes #1007

This allows updating code in both places, if necessary. Getting Django
1.5 support would be painful otherwise. This also removes the circular
dependency of django-oscar-testsupport on Oscar.

The requirements were merged. The imports were updated
accordingly. Unused imports in the touched files were removed. No
further changes.

Fixes #376

Also split them into unit and functional tests.