pytest: fix flake in test_channel_lease_post_expiry

We close the channel because the payment fulfilment is not totally done, and
we generate 6 blocks, causing it to hit CLTV deadline.

```
        # send some payments, mine a block or two
        inv = l2.rpc.invoice(10**4, '1', 'no_1')
        l1.rpc.pay(inv['bolt11'])
    
        # l2 attempts to close a channel that it leased, should fail
        with pytest.raises(RpcError, match=r'Peer leased this channel from us'):
            l2.rpc.close(l1.get_channel_scid(l2))
    
        bitcoind.generate_block(6)
        sync_blockheight(bitcoind, [l1, l2])
        # make sure we're at the right place for the csv lock
>       l2.daemon.wait_for_log('Blockheight: SENT_ADD_ACK_COMMIT->RCVD_ADD_ACK_REVOCATION LOCAL now 115')

tests/test_closing.py:823: 
...
lightningd-2 2022-07-17T13:15:34.242Z DEBUG   lightningd: Adding block 115: 39d95061935e9fc42b04c86ae60d0cf157765aff4c040f3a8d0b7888db19e015
lightningd-2 2022-07-17T13:15:34.244Z UNUSUAL 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#2: Peer permanent failure in CHANNELD_NORMAL: Fulfilled HTLC 0 SENT_REMOVE_COMMIT cltv 115 hit deadline
```

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

tests/test_closing.py

@@ -813,6 +813,10 @@ def test_channel_lease_post_expiry(node_factory, bitcoind, chainparams):
     inv = l2.rpc.invoice(10**4, '1', 'no_1')
     l1.rpc.pay(inv['bolt11'])
 
+    # make sure it's completely resolved before we generate blocks,
+    # otherwise it can close HTLC!
+    wait_for(lambda: only_one(only_one(l2.rpc.listpeers()['peers'])['channels'])['htlcs'] == [])
+
     # l2 attempts to close a channel that it leased, should fail
     with pytest.raises(RpcError, match=r'Peer leased this channel from us'):
         l2.rpc.close(l1.get_channel_scid(l2))