Add backupServer1 and backupServer2

Two new CDNServerMux instances which will act as our failover backends. I've
used the word "backup" instead of "mirror" because it's more generic and not
particular to our setup (not everyone will have a static mirror).
Move/split/rename error page test

These probably belong to the TestFailover "category". I also suspect that
the behaviour will be different depending on whether all backends are down
or returning 5xx responses, so I've split the test in two.
Varnish mocks for backupServer{1,2} failover

This is an abbreviated copy of the logic in gds/fastly-config.
Add empty TestFailoverOrigin5xxBackOff

As implemented with `beresp.saintmode` in:

https://github.gds/gds/fastly-config/commit/c1d5081e59844a2b64e11cd0d062a6ce6b53c647
Merge pull request #43 from alphagov/failover_servers_and_mocks

Mocks and additional tests for failover