pytest: make line_graph wait for gossip propagation if announce param is True

This is what the callers want; generalize it.

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

tests/utils.py

@@ -804,15 +804,27 @@ class NodeFactory(object):
 
         # Confirm all channels and wait for them to become usable
         bitcoin.generate_block(1)
+        scids = []
         for src, dst in connections:
             wait_for(lambda: src.channel_state(dst) == 'CHANNELD_NORMAL')
             scid = src.get_channel_scid(dst)
             src.daemon.wait_for_log(r'Received channel_update for channel {scid}\(.\) now ACTIVE'.format(scid=scid))
+            scids.append(scid)
 
         if not announce:
             return nodes
 
         bitcoin.generate_block(5)
+
+        def both_dirs_ready(n, scid):
+            resp = n.rpc.listchannels(scid)
+            return [a['active'] for a in resp['channels']] == [True, True]
+
+        # Make sure everyone sees all channels: we can cheat and
+        # simply check the ends (since it's a line).
+        wait_for(lambda: both_dirs_ready(nodes[0], scids[-1]))
+        wait_for(lambda: both_dirs_ready(nodes[-1], scids[0]))
+
         return nodes
 
     def killall(self, expected_successes):