pytest: fix flake in test_splicing.py::test_route_by_old_scid

```
2025-09-30T00:07:37.3118831Z _________________ ERROR at teardown of test_route_by_old_scid __________________
...
2025-09-30T00:07:37.3131523Z             request.node.has_errors = True
2025-09-30T00:07:37.3131989Z >           raise ValueError(str(errors))
2025-09-30T00:07:37.3132834Z E           ValueError: 
2025-09-30T00:07:37.3133226Z E           Node errors:
2025-09-30T00:07:37.3133632Z E            - lightningd-1: had warning messages
2025-09-30T00:07:37.3134086Z E           Global errors:
...
2025-09-30T00:07:37.5835258Z lightningd-2 2025-09-30T00:04:24.544Z TRACE   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-gossipd: Bad gossip order: channel_announcement: no unspent txout 109x1x1
2025-09-30T00:07:37.5835659Z lightningd-2 2025-09-30T00:04:24.544Z DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-connectd: peer_out WIRE_WARNING
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tests/test_splicing.py

@@ -527,6 +527,11 @@ def test_route_by_old_scid(node_factory, bitcoind):
     l1.rpc.sendpay(route, inv['payment_hash'], payment_secret=inv['payment_secret'])
     l1.rpc.waitsendpay(inv['payment_hash'])
 
+    # Make sure l1 has seen and processed announcement for new splice
+    # scid, otherwise we can get gossip warning here (which breaks CI) if we splice again.
+    scid = only_one(l3.rpc.listchannels(source=l3.info['id'])['channels'])['short_channel_id']
+    wait_for(lambda: l1.rpc.listchannels(short_channel_id=scid)['channels'] != [])
+
     # Let's splice again, so the original scid is two behind the times.
     l3.fundwallet(200000)
     funds_result = l3.rpc.fundpsbt("109000sat", "slow", 166, excess_as_change=True)