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