pytest: avoid bad gossip flake in test_htlc_no_force_close

l3 closes with l2 unilaterally, but l2 sends l1 the channel_announce after
that, when it has never seen it before.  The solution is to wait until
l1 has seen the channel before we start the rest of the test:

```
E           ValueError:
E           Node errors:
E            - lightningd-2: had warning messages
E            - lightningd-1: had bad gossip messages
...
lightningd-2 2025-07-09T07:55:59.778Z DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-connectd: peer_in WIRE_WARNING
lightningd-2 2025-07-09T07:55:59.778Z INFO    0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-connectd: Received WIRE_WARNING: WARNING: channel_announcement: no unspent txout 103x1x0
lightningd-1 2025-07-09T07:55:59.781Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-channeld-chan#1: billboard: Channel ready for use.
lightningd-1 2025-07-09T07:55:59.782Z TRACE   gossipd: channel_announcement: got reply for 103x1x0...
lightningd-1 2025-07-09T07:55:59.782Z TRACE   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-gossipd: Bad gossip order: channel_announcement: no unspent txout 103x1x0
```

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

Author: rustyrussell

tests/test_closing.py

@@ -3882,7 +3882,7 @@ def test_htlc_no_force_close(node_factory, bitcoind, anchors):
         for opt in opts:
             opt['dev-force-features'] = "-23"
 
-    l1, l2, l3 = node_factory.line_graph(3, opts=opts)
+    l1, l2, l3 = node_factory.line_graph(3, opts=opts, wait_for_announce=True)
 
     MSATS = 12300000
     inv = l3.rpc.invoice(MSATS, 'label', 'description')