pytest: fix test_gossip_no_empty_announcements flake.

This is a side-effect of fixing aging: sometimes, we age our
rcvd_filter cache too fast, and thus re-xmit.  This breaks
our test, since it used dev-disconnect on the channel_announce,
but that closes to l3, not l1!

```
>       assert l1.rpc.listchannels()['channels'] == []
E       AssertionError: assert [{'active': T...ags': 1, ...}] == []
E         Left contains 2 more items, first extra item: {'active': True, 'amount_msat': 100000000msat, 'base_fee_millisatoshi': 1, 'channel_flags': 0, ...}
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Fixes: #5403

tests/test_gossip.py

@@ -622,10 +622,10 @@ def test_gossip_no_empty_announcements(node_factory, bitcoind, chainparams):
     # l2 sends CHANNEL_ANNOUNCEMENT to l1, then disconnects/
     l2.daemon.wait_for_log('dev_disconnect')
     l1.daemon.wait_for_log(r'\[IN\] 0100')
+    wait_for(lambda: l1.rpc.listchannels()['channels'] == [])
 
     # l1 won't relay it (make sure it has time to digest though)
     time.sleep(2)
-    assert l1.rpc.listchannels()['channels'] == []
     encoded = subprocess.run(['devtools/mkencoded', '--scids', '00'],
                              check=True,
                              timeout=TIMEOUT,