Fix test behaviour on Django < 1.6.

* While it may have appeared that django-admin-sso was
  extremely thoroughly tested, with a battery of 484
  tests, all but 6 of those were Django's or OpenID's -
  i.e. those of django-admin-sso's dependencies.
* DiscoverRunner can fix this for us, but it's not
  built into Django < 1.6, so we need to manually
  import it.  The input hacking has been isolated
  in a test runner file (which also makes it easier
  to expand the runner later if we so desire).
* Move tests.py into the newly created test folder
  alongside the test runner and rename it to
  something more descriptive.
Merge pull request #1 from incuna/i01-fix-tests

Fix test behaviour in Django <1.6