Use 'GET /?cacheBuster=%s' for origin check req
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.

mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests