More failover tests

Implement:

- TestFailoverErrorPageAllServers5xx
- TestFailoverOriginDownUseFirstMirror
- TestFailoverOriginDownFirstMirrorDownUseSecondMirror

Passing against the mocks and the real CDN:
```
dcarley@ci-slave-4:~/cdn-acceptance-tests$ go test -edgeHost fastly.cdn-acceptance-test.alphagov.co.uk -skipVerifyTLS -v -run 'TestFailoverErrorPageAllServers5xx|TestFailoverOriginDownUseFirstMirror|TestFailoverOriginDownFirstMirrorDownUseSecondMirror'
2014/06/26 08:13:07 Confirming that CDN is healthy
2014/06/26 08:13:07 Started server on port 8082
2014/06/26 08:13:07 Checking health of backup2...
2014/06/26 08:13:15 backup2 is up!
2014/06/26 08:13:15 Started server on port 80812014/06/26 08:13:15 Checking health of backup1...
2014/06/26 08:13:30 backup1 is up!
2014/06/26 08:13:30 Started server on port 8080
2014/06/26 08:13:30 Checking health of origin...
2014/06/26 08:13:41 origin is up!
=== RUN TestFailoverErrorPageAllServers5xx
--- PASS: TestFailoverErrorPageAllServers5xx (0.05 seconds)
=== RUN TestFailoverOriginDownUseFirstMirror
--- PASS: TestFailoverOriginDownUseFirstMirror (0.05 seconds)
=== RUN TestFailoverOriginDownFirstMirrorDownUseSecondMirror
2014/06/26 08:13:41 Started server on port 8080
2014/06/26 08:13:41 Checking health of origin...
2014/06/26 08:13:41 origin is up!
--- PASS: TestFailoverOriginDownFirstMirrorDownUseSecondMirror (0.04 seconds)
PASS
ok      _/home/dcarley/cdn-acceptance-tests     33.471s
```
Ah, gotcha. So we're essentially testing that we don't failover 'too far' when we receive 5XX statuses from all backends.