pytest: fix flake in test_fetchinvoice

The sleep was simply allowing gossip to propagate, so (sometimes) l4 can connect to l3/l1.
We really want to suppress the blinded path on l2:

```
2025-03-03T05:25:16.6928072Z >       l4.rpc.call('fetchinvoice', {'offer': offer3['bolt12']})
2025-03-03T05:25:16.6928280Z
2025-03-03T05:25:16.6928367Z tests/test_pay.py:4540:
2025-03-03T05:25:16.6928609Z _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
2025-03-03T05:25:16.6928929Z contrib/pyln-testing/pyln/testing/utils.py:760: in call
...
2025-03-03T05:25:16.6956050Z E           pyln.client.lightning.RpcError: RPC call failed: method: fetchinvoice, payload: {'offer': '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'}, error: {'code': 1003, 'message': 'Failed: could not route or connect directly to blinded path at 035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d: {"code":400,"message":"Unable to connect, no address known for peer"}'}
```

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

Author: rustyrussell

tests/test_pay.py

@@ -4437,7 +4437,7 @@ def test_fetchinvoice(node_factory, bitcoind):
     # We remove the conversion plugin on l3, causing it to get upset.
     l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True,
                                          opts=[{},
-                                               {},
+                                               {'dev-allow-localhost': None},
                                                {'broken_log': "plugin-offers: Failed invreq.*Unknown command 'currencyconvert'",
                                                 'dev-allow-localhost': None}])
 
@@ -4535,7 +4535,6 @@ def test_fetchinvoice(node_factory, bitcoind):
                                    'description': 'offer3'})
     l4 = node_factory.get_node()
     l4.rpc.connect(l2.info['id'], 'localhost', l2.port)
-    time.sleep(0.25)
     # ... even if we can't find ourselves.
     l4.rpc.call('fetchinvoice', {'offer': offer3['bolt12']})
     # ... even if we know it from gossmap