Add an async-payment-via-LSPS2 integration test

Test the full end-to-end flow of an async payment through an LSPS2 JIT
channel: static invoice server setup, LSPS2BOLT12Router registration,
async payment onion message exchange (HeldHtlcAvailable/ReleaseHeldHtlc),
HTLC interception, JIT channel open, and payment claim.

Co-Authored-By: HAL 9000
Signed-off-by: Elias Rohrer <dev@tnull.de>

Author: tnull