pytest: try to fix flake in test_lightningd_still_loading

I can't reproduce this, but CI did (with Elements):

```
[gw3] linux -- Python 3.8.18 /home/runner/.cache/pypoetry/virtualenvs/cln-meta-project-AqJ9wMix-py3.8/bin/python

node_factory = <pyln.testing.utils.NodeFactory object at 0x7fd0e20f57f0>
bitcoind = <pyln.testing.utils.ElementsD object at 0x7fd0e307dbe0>
executor = <concurrent.futures.thread.ThreadPoolExecutor object at 0x7fd0e307da30>

    @pytest.mark.openchannel('v1')
    @pytest.mark.openchannel('v2')
    def test_lightningd_still_loading(node_factory, bitcoind, executor):
        """Test that we recognize we haven't got all blocks from bitcoind"""
    
        mock_release = Event()
    
        # This is slow enough that we're going to notice.
        def mock_getblock(r):
            conf_file = os.path.join(bitcoind.bitcoin_dir, 'bitcoin.conf')
            brpc = RawProxy(btc_conf_file=conf_file)
            if r['params'][0] == slow_blockid:
                mock_release.wait(TIMEOUT)
            return {
                "result": brpc._call(r['method'], *r['params']),
                "error": None,
                "id": r['id']
            }
    
        # Start it, establish channel, get extra funds.
        l1, l2, l3 = node_factory.get_nodes(3, opts=[{'may_reconnect': True,
                                                      'wait_for_bitcoind_sync': False},
                                                     {'may_reconnect': True,
                                                      'wait_for_bitcoind_sync': False},
                                                     {}])
        node_factory.join_nodes([l1, l2])
    
        # Balance l1<->l2 channel
        l1.pay(l2, 10**9 // 2)
    
        l1.stop()
    
        # Now make sure l2 is behind.
        bitcoind.generate_block(2)
        # Make sure l2/l3 are synced
        sync_blockheight(bitcoind, [l2, l3])
    
        # Make it slow grabbing the final block.
        slow_blockid = bitcoind.rpc.getblockhash(bitcoind.rpc.getblockcount())
        l1.daemon.rpcproxy.mock_rpc('getblock', mock_getblock)
    
        l1.start(wait_for_bitcoind_sync=False)
    
        # It will warn about being out-of-sync.
        assert 'warning_bitcoind_sync' not in l1.rpc.getinfo()
        assert 'warning_lightningd_sync' in l1.rpc.getinfo()
    
        # Make sure it's connected to l2 (otherwise we get TEMPORARY_CHANNEL_FAILURE)
        wait_for(lambda: only_one(l1.rpc.listpeers(l2.info['id'])['peers'])['connected'])
    
        # Payments will succced.
        l1.pay(l2, 1000)
>       assert l1.daemon.is_in_log(r"Sending HTLC while still syncing with bitcoin network \(104 vs 105\)")
E       AssertionError: assert None
E        +  where None = <bound method TailableProc.is_in_log of <pyln.testing.utils.LightningD object at 0x7fd0e20f9fa0>>('Sending HTLC while still syncing with bitcoin network \\(104 vs 105\\)')
E        +    where <bound method TailableProc.is_in_log of <pyln.testing.utils.LightningD object at 0x7fd0e20f9fa0>> = <pyln.testing.utils.LightningD object at 0x7fd0e20f9fa0>.is_in_log
E        +      where <pyln.testing.utils.LightningD object at 0x7fd0e20f9fa0> = <fixtures.LightningNode object at 0x7fd0e20f59d0>.daemon
```

What was in logs was:

```
lightningd-1 2024-11-18T05:33:50.634Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-chan#1: Sending HTLC while still syncing with bitcoin network (103 vs 105)
```

Implying that l1 was an extra block behind.

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

tests/test_misc.py

@@ -283,6 +283,7 @@ def test_lightningd_still_loading(node_factory, bitcoind, executor):
 
     # Make sure it's connected to l2 (otherwise we get TEMPORARY_CHANNEL_FAILURE)
     wait_for(lambda: only_one(l1.rpc.listpeers(l2.info['id'])['peers'])['connected'])
+    wait_for(lambda: l1.rpc.getinfo()['blockheight'] == 104)
 
     # Payments will succced.
     l1.pay(l2, 1000)