tests/hardened: fix test for slower hardware

[erikarvstedt]

Due to the slowdown caused by the hardening features, the hardened test occasionally failed since the upgrade to NixOS 23.11.
Error:

vm-test-run-nix-bitcoin-hardened> machine # [  147.136056] lightningd[1667]: UNUSUAL plugin-cl-zmq.py: Killing plugin: timed out before replying to getmanifest
vm-test-run-nix-bitcoin-hardened> machine # [  147.151591] lightningd[1667]: UNUSUAL plugin-cl-zmq.py: Killing plugin: Invalid/missing result tok in '{"jsonrpc": "2.0","id": 16,"error": {"code":-4, "message":"Plugin terminated before replying to RPC call."}}'
vm-test-run-nix-bitcoin-hardened> machine # [  147.152555] lightningd[1667]: Can't recover from plugin failure, terminating.`

[jonasnick]

utACK 98b16a1