pytest: fix flake in test_pay_disconnect

If channeld hasn't exited yet, it's possible we'll send the message (we would fail later, in
waitsendpay, but just not immediately).  So wait for that explicitly.

```
2022-09-22T22:49:59.6737296Z         with pytest.raises(RpcError, match=r'failed: WIRE_TEMPORARY_CHANNEL_FAILURE \(First peer not ready\)'):
2022-09-22T22:49:59.6737566Z >           l1.rpc.sendpay(route, rhash, payment_secret=inv['payment_secret'])
2022-09-22T22:49:59.6737865Z E           Failed: DID NOT RAISE <class 'pyln.client.lightning.RpcError'>
2022-09-22T22:49:59.6737873Z 
```

And from the listpeers output, ou can see "connected" false, but owner channeld:

```
2022-09-22T22:49:59.7493163Z DEBUG:root:{
2022-09-22T22:49:59.7493320Z   "id": "-c:listpeers#26",
2022-09-22T22:49:59.7493397Z   "result": {
2022-09-22T22:49:59.7493477Z     "peers": [
2022-09-22T22:49:59.7493548Z       {
2022-09-22T22:49:59.7493709Z         "id": "022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59",
2022-09-22T22:49:59.7493801Z         "connected": false,
2022-09-22T22:49:59.7493884Z         "channels": [
2022-09-22T22:49:59.7493955Z           {
2022-09-22T22:49:59.7494058Z             "state": "CHANNELD_NORMAL",
2022-09-22T22:49:59.7494250Z             "scratch_txid": "4b95a3b1b5e1a970401a169a3697f3a9bfbfbcb59d3d21434aa1f3fb2980db8d",
2022-09-22T22:49:59.7494365Z             "last_tx_fee_msat": "7965000msat",
2022-09-22T22:49:59.7494437Z             "feerate": {
2022-09-22T22:49:59.7494529Z               "perkw": 11000,
2022-09-22T22:49:59.7494618Z               "perkb": 44000
2022-09-22T22:49:59.7494690Z             },
2022-09-22T22:49:59.7494785Z             "owner": "channeld",
2022-09-22T22:49:59.7494894Z             "short_channel_id": "103x1x0",
```

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

tests/test_pay.py

@@ -266,7 +266,8 @@ def test_pay_disconnect(node_factory, bitcoind):
     route = l1.rpc.getroute(l2.info['id'], 123000, 1)["route"]
 
     l2.stop()
-    wait_for(lambda: only_one(l1.rpc.listpeers(l2.info['id'])['peers'])['connected'] is False)
+    # Make sure channeld has exited!
+    wait_for(lambda: 'owner' not in only_one(only_one(l1.rpc.listpeers(l2.info['id'])['peers'])['channels']))
 
     # Can't pay while its offline.
     with pytest.raises(RpcError, match=r'failed: WIRE_TEMPORARY_CHANNEL_FAILURE \(First peer not ready\)'):