REGTESTS: filters: Exercise Lua callback resumption

REGTESTS: filters: Exercise Lua callback resumption

Chain waiting and non-waiting Lua filters on the frontend and backend,
with different global, request and response orders. Return WAIT once in
start/header callbacks and verify that resumption does not duplicate
headers, recreate instances or lose the independently ordered lists.

Combine the waits with small payload forwarding quotas, keep-alive and
chunked request/response bodies. Use a barrier to keep the backend open
until the client drains the final response; EOF handling is independent
of the resumption behavior checked here.