REGTESTS: filters: Exercise Lua filters across HTTP retries

REGTESTS: filters: Exercise Lua filters across HTTP retries

Retry a 503 response and redispatch to a second server. Check that both
servers receive the same request header order and instance identifiers,
without another attachment or duplicate request processing.

Verify that the failed response does not leak headers into the final
response, and that its filter order and instances still follow the
frontend/backend response sequences. Include a waiting frontend filter.