pytest: fix flake in test_gossip_force_broadcast_channel_msgs

We can get more gossip_filter messages now.  And we can also go over max-messages,
so increase that too.

```
        del tally['query_short_channel_ids']
        del tally['query_channel_range']
        del tally['ping']
>       assert tally == {'channel_announce': 1,
                         'channel_update': 3,
                         'node_announce': 1,
                         'gossip_filter': 1}
E       AssertionError: assert {'channel_ann..._announce': 1} == {'channel_ann..._announce': 1}
E         Omitting 2 identical items, use -vv to show
E         Differing items:
E         {'gossip_filter': 2} != {'gossip_filter': 1}
E         {'channel_update': 2} != {'channel_update': 3}
E         Full diff:
E           {
E            'channel_announce': 1,...
E         
E         ...Full output truncated (10 lines hidden), use '-vv' to show

tests/test_gossip.py:2326: AssertionError
```

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

tests/test_gossip.py

@@ -2310,7 +2310,7 @@ def test_gossip_force_broadcast_channel_msgs(node_factory, bitcoind):
                             '--no-gossip',
                             '--hex',
                             '--network={}'.format(TEST_NETWORK),
-                            '--max-messages={}'.format(7),
+                            '--max-messages={}'.format(10),
                             '--timeout-after={}'.format(120),
                             '{}@localhost:{}'.format(l1.info['id'], l1.port)],
                            check=True,
@@ -2320,10 +2320,11 @@ def test_gossip_force_broadcast_channel_msgs(node_factory, bitcoind):
     for l in lines:
         tally[types[l[0:4]]] += 1
 
+    assert tally['gossip_filter'] >= 1
     del tally['query_short_channel_ids']
     del tally['query_channel_range']
     del tally['ping']
+    del tally['gossip_filter']
     assert tally == {'channel_announce': 1,
                      'channel_update': 3,
-                     'node_announce': 1,
-                     'gossip_filter': 1}
+                     'node_announce': 1}