Add test for X-Cache-Hits behaviour
Add test for X-Cache-Hits behaviour

Slightly different to what we originally expected, the Edge appends its
own hit count to the X-Cache-Hits header.

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