pytest: Attempting to fix the flaky gossip test

This seems to happen when we manage to check between the
channel_announcement and the channel_update being processed.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

tests/test_lightningd.py

@@ -1285,7 +1285,7 @@ class LightningDTests(BaseLightningDTests):
 
         channels = l1.rpc.getchannels()['channels']
         assert len(channels) == 2
-        assert [c['active'] for c in channels] == [True, True]
+        wait_for(lambda: [c['active'] for c in channels] == [True, True])
 
     def ping_tests(self, l1, l2):
         # 0-byte pong gives just type + length field.