Add requirements-test.txt

Add requirements-test.txt

It's annoying that Django is not installed by default when setting up
a development environment. This is currently necessary because tox
uses these requirements files and it needs to test against multiple
versions of Django. Resolve this issue by adding a 'requirements-test'
file, thus allowing for Django-ful and Django-less scenarios.

This also allows us to delete 'requirements-base', which was a good
idea but alas contained too few options to really be viable. A little
duplication is better here.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>