pytest: fix flake in test_wumbo_channels

We mine blocks too fast, and l3 discard the channel_announcment as too far in the future:

```
lightningd-3 2023-12-14T06:40:04.744Z DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-gossipd: Ignoring future channel_announcment for 103x1x1 (current block 102)
```

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

tests/test_connection.py

@@ -3470,6 +3470,8 @@ def test_wumbo_channels(node_factory, bitcoind):
     # Get that mined, and announced.
     bitcoind.generate_block(6, wait_for_mempool=1)
 
+    # Make sure l3 is ready to receive channel announcement!
+    sync_blockheight(bitcoind, [l1, l2, l3])
     # Connect l3, get gossip.
     l3.rpc.connect(l1.info['id'], 'localhost', port=l1.port)