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'}
```
9758b05bd0