pytest: fix flake in test_route_by_old_scid

```
2025-08-14T11:45:41.7353946Z         # Now l1 tries to send using old scid: should work
2025-08-14T11:45:41.7354652Z         l1.rpc.sendpay(route, inv['payment_hash'], payment_secret=inv['payment_secret'])
2025-08-14T11:45:41.7355321Z >       l1.rpc.waitsendpay(inv['payment_hash'])
2025-08-14T11:45:41.7355644Z 
2025-08-14T11:45:41.7355791Z tests/test_splicing.py:528: 
...
2025-08-14T11:45:41.7383073Z E           pyln.client.lightning.RpcError: RPC call failed: method: waitsendpay, payload: {'payment_hash': '7b74fa9f6a889a16ebf89b8a9468302100f6ad50a771bbab2a16de58dcb1a9a4'}, error: {'code': 203, 'message': 'failed: WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS (reply from remote)', 'data': {'created_index': 1, 'id': 1, 'payment_hash': '7b74fa9f6a889a16ebf89b8a9468302100f6ad50a771bbab2a16de58dcb1a9a4', 'groupid': 1, 'destination': '035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d', 'amount_msat': 10000000, 'amount_sent_msat': 10000101, 'created_at': 1755171392, 'status': 'pending', 'erring_index': 2, 'failcode': 16399, 'failcodename': 'WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS', 'erring_node': '035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d', 'erring_channel': '103x2x0', 'erring_direction': 0, 'raw_message': '400f000000000098968000000072'}}
```

Caused by:

```
2025-08-14T11:45:41.9275961Z lightningd-3 2025-08-14T11:36:32.700Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-chan#1: Expiry cltv too soon 118 < 114 + 5
```

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

Author: rustyrussell

tests/test_splicing.py

@@ -506,7 +506,7 @@ def test_route_by_old_scid(node_factory, bitcoind):
     # Get pre-splice route.
     inv = l3.rpc.invoice(10000000, 'test_route_by_old_scid', 'test_route_by_old_scid')
     inv2 = l3.rpc.invoice(10000000, 'test_route_by_old_scid2', 'test_route_by_old_scid2')
-    route = l1.rpc.getroute(l3.info['id'], 10000000, 1)['route']
+    route = l1.rpc.getroute(l3.info['id'], 10000000, 1, cltv=16)['route']
 
     # Do a splice
     funds_result = l2.rpc.fundpsbt("109000sat", "slow", 166, excess_as_change=True)