Test for stagecraft `/data-sets` now expects a 403

Since we protected the `/data-sets` endpoint with a token, we should now
expect to get a 403 rather than a 200 when we hit the `/data-sets`
endpoint.

This tells us that we've managed to route through to the app.

See https://www.pivotaltracker.com/story/show/67103208

[#67103208]
Merge pull request #5 from alphagov/data-sets-test-now-expects-403

Test for stagecraft `/data-sets` now expects a 403