Implement TestFailoverErrorPageAllServers5xx
Implement TestFailoverErrorPageAllServers5xx

The behaviour differs a little from the original description, in that the
last 5xx response is delivered instead of Varnish's internal error. But
that's actually to be expected and is a good thing.

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