pytest: fix flake in test_penalty_htlc_tx_fulfill

Make sure balancing payment is fully cleared before trying to get a routeL

```
    def test_penalty_htlc_tx_fulfill(node_factory, bitcoind, chainparams, anchors):

        # now we send one 'sticky' htlc: l4->l1
        amt = 10**8 // 2
        sticky_inv = l1.rpc.invoice(amt, '2', 'sticky')
>       route = l4.rpc.getroute(l1.info['id'], amt, 1)['route']

tests/test_closing.py:1232:

>           raise RpcError(method, payload, resp['error'])
E           pyln.client.lightning.RpcError: RPC call failed: method: getroute, payload: {'id': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518', 'amount_msat': 50000000, 'riskfactor': 1, 'cltv': 9}, error: {'code': 205, 'message': 'Could not find a route'}
```

tests/test_closing.py

@@ -1221,10 +1221,12 @@ def test_penalty_htlc_tx_fulfill(node_factory, bitcoind, chainparams, anchors):
     inv = l2.rpc.invoice(10**9 // 2, '1', 'balancer')
     l1.rpc.pay(inv['bolt11'])
     l1.rpc.waitsendpay(inv['payment_hash'])
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels()['channels'])['htlcs'] == [])
 
     inv = l4.rpc.invoice(10**9 // 2, '1', 'balancer')
     l2.rpc.pay(inv['bolt11'])
     l2.rpc.waitsendpay(inv['payment_hash'])
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels()['channels'])['htlcs'] == [])
 
     # now we send one 'sticky' htlc: l4->l1
     amt = 10**8 // 2