Add complete test for cached responses

Ensure that the CDN is behaving as CDNs should. Response to first request is
cached and delivered on subsequent requests without hitting origin.
Adjust attempt and wait durations

* Sleep for less time, check more often.

* Assume that 'ok on first attempt' means CDN already has origin marked
  as ok.

* Also adjust loop logic so that we aren't checking maxRetries twice.
Use 'GET /?cacheBuster=%s' for origin check req

This allows us to run the tests against a live environment, as
a 200 will be returned, but the origin will still need to be checked.
Merge pull request #5 from alphagov/simple_cache_test

Add complete test for cached responses
Merge pull request #9 from alphagov/origin_check_improve

Improve confirmOriginIsEnabled
Implement test for PURGE returning 403

This test, rather excitingly, proves that the behaviour has changed since we
implemented it in: https://www.pivotaltracker.com/story/show/62136616

The CDN now returns 200 and the body of a normal GET request. I suspect that
it's something to do with the `req.request = GET` hack that they suggested
implementing.

We should merge this but separately fix the config to make this test pass.