pytest: fix flake in test_blindedpath_privchan.

We reconnect from l3->l2, but l2 is also trying to reconnect and
we can race:

```
        # Now try when l3 uses scid for entry point of blinded path.
        l3.stop()
        l3.daemon.opts['dev-invoice-bpath-scid'] = None
        l3.start()
>       l3.rpc.connect(l2.info['id'], 'localhost', l2.port)

tests/test_pay.py:5667:
...
>           raise RpcError(method, payload, resp['error'])
E           pyln.client.lightning.RpcError: RPC call failed: method: connect, payload: {'id': '022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59', 'host': 'localhost', 'port': 33557}, error: {'code': 402, 'message': 'disconnected during connection'}
```

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

Author: rustyrussell

tests/test_pay.py

@@ -5664,7 +5664,7 @@ def test_blindedpath_privchan(node_factory, bitcoind):
     l3.stop()
     l3.daemon.opts['dev-invoice-bpath-scid'] = None
     l3.start()
-    l3.rpc.connect(l2.info['id'], 'localhost', l2.port)
+    wait_for(lambda: [p['connected'] for p in l3.rpc.listpeers(l2.info['id'])['peers']] == [True])
 
     chan = only_one(l1.rpc.listchannels(source=l2.info['id'])['channels'])