pytest: fix flake in test_onionmessage_forward_fail

We can fail to connect because l3 connects to l2 at the same time l2 connects to l3:

```
        # The plugin in l2 fixes up the connection, so this works!
>       l1.rpc.fetchinvoice(offer['bolt12'])

tests/test_connection.py:4792:
...
        elif "error" in resp:
>           raise RpcError(method, payload, resp['error'])
E           pyln.client.lightning.RpcError: RPC call failed: method: fetchinvoice, payload: {'offer': 'lno1qgsqvgnwgcg35z6ee2h3yczraddm72xrfua9uve2rlrm9deu7xyfzrcgqgqjczs7w3jhxazldahxjmmwd4jhxumpvaj47en0wfmkzunytanxz6tvzcssxhftzxfdlwsnfcgw2sy8t5mxa0ytcdfat2nkdwqvpy9nnsa9mzza'}, error: {'code': 1005, 'message': 'Timeout waiting for response'}
```

From logs:

```
lightningd-2 2025-05-07T21:20:54.367Z DEBUG   035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d-connectd: peer_out WIRE_INIT
lightningd-2 2025-05-07T21:20:54.367Z DEBUG   035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d-connectd: Connect IN
``

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

Author: rustyrussell

tests/test_connection.py

@@ -4780,9 +4780,11 @@ def test_onionmessage_forward_fail(node_factory, bitcoind):
                                          opts=[{},
                                                {'dev-allow-localhost': None,
                                                 'may_reconnect': True,
+                                                'dev-no-reconnect': None,
                                                 'plugin': os.path.join(os.getcwd(), 'tests/plugins/onionmessage_forward_fail_notification.py'),
                                                 },
                                                {'dev-allow-localhost': None,
+                                                'dev-no-reconnect': None,
                                                 'may_reconnect': True}])
 
     offer = l3.rpc.offer(300, "test_onionmessage_forward_fail")