Add empty TestFailoverOrigin5xxBackOff
Add empty TestFailoverOrigin5xxBackOff

As implemented with `beresp.saintmode` in:

https://github.gds/gds/fastly-config/commit/c1d5081e59844a2b64e11cd0d062a6ce6b53c647

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