[Fixes #74035692 #63] Implement TestFailoverOrigin5xxBackOff

EOM.
The handler gets switched on request 2 and stays that way for all remaining requests in the loop.

There's another test in this file where we switch the handler and expected response, loop through a few requests, and then switch again: https://github.com/alphagov/cdn-acceptance-tests/blob/74035692-implement_5xx_backoff/cdn_failover_test.go#L203-211

Does that make sense? Could it be clarified in any way?