pytest: fix flake in test_bolt11_null_after_pay

It's possible that l2 hasn't completely processed the connection yet:

```
        # create l2->l1 channel.
        l2.fundwallet(amount_sat * 5)
        l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
>       l2.rpc.fundchannel(l1.info['id'], amount_sat * 3)

tests/test_pay.py:3974:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
contrib/pyln-client/pyln/client/lightning.py:833: in fundchannel
    return self.call("fundchannel", payload)
contrib/pyln-testing/pyln/testing/utils.py:706: in call
    res = LightningRpc.call(self, method, payload, cmdprefix, filter)
...
E           pyln.client.lightning.RpcError: RPC call failed: method: fundchannel, payload: {'id': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518', 'amount': 3000000, 'announce': True}, error: {'code': 400, 'message': 'Unable to connect, no address known for peer', 'data': {'id': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518', 'method': 'connect'}}

contrib/pyln-client/pyln/client/lightning.py:422: RpcError
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_pay.py

@@ -3971,6 +3971,8 @@ def test_bolt11_null_after_pay(node_factory, bitcoind):
     # create l2->l1 channel.
     l2.fundwallet(amount_sat * 5)
     l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
+    # Make sure l2 considers it fully connected too!
+    wait_for(lambda: l2.rpc.listpeers(l1.info['id']) != {'peers': []})
     l2.rpc.fundchannel(l1.info['id'], amount_sat * 3)
 
     # Let the channel confirm.