Remove references to static in tests

- static.preview.alphagov.co.uk points to an IP that should not be
  publicly available and we will remove.
- assets-origin.preview.alphagov.co.uk points to an IP that is publicly
  available and will remain.

In order that the tests succeed when this DNS/IP is removed, we need to
update the tests.
Merge pull request #647 from alphagov/remove-static-from-tests

Remove references to static in tests