Stagecraft now redirects `/admin` properly.

- Remove the @knownfailing tag so it actually runs
- Update the expected 301 location to the full preview/production
  location

  See https://www.pivotaltracker.com/story/show/67097186

  [#67097186]
Merge pull request #4 from alphagov/stagecraft-redirect-test-fixed

Stagecraft now redirects `/admin` properly.