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.
Implement TestFailoverOriginDownUseFirstMirror.
Implement OriginDownFirstMirrorDownUseSecondMirror
Merge pull request #58 from alphagov/more_failover_tests

More failover tests