REGTESTS: filters: Exercise per-thread Lua instance state

REGTESTS: filters: Exercise per-thread Lua instance state

Load the HTTP probes separately on each thread and inherit frontend
instances into listeners pinned to different threads. Check the Lua
state id and per-state construction counters through frontend and backend
callbacks, including a second stream on each thread.

Verify that instances, disabled instances and independent response
sequences are handled correctly without sharing mutable Lua state between
threads. Extend the probe headers with the Lua state id.