pytest: fix test in test_gossip_pruning

It's possible that listchannels doesn't show the channel yet:

```
    
        l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
        l2.rpc.connect(l3.info['id'], 'localhost', l3.port)
    
        scid1, _ = l1.fundchannel(l2, 10**6)
        scid2, _ = l2.fundchannel(l3, 10**6)
    
        mine_funding_to_announce(bitcoind, [l1, l2, l3])
>       l1_initial_cupdate_timestamp = only_one(l1.rpc.listchannels(source=l1.info['id'])['channels'])['last_update']

tests/test_gossip.py:43: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

arr = []

    def only_one(arr):
        """Many JSON RPC calls return an array; often we only expect a single entry
        """
>       assert len(arr) == 1
E       AssertionError
```

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

Author: rustyrussell

tests/test_gossip.py

@@ -40,6 +40,7 @@ def test_gossip_pruning(node_factory, bitcoind):
     scid2, _ = l2.fundchannel(l3, 10**6)
 
     mine_funding_to_announce(bitcoind, [l1, l2, l3])
+    wait_for(lambda: l1.rpc.listchannels(source=l1.info['id'])['channels'] != [])
     l1_initial_cupdate_timestamp = only_one(l1.rpc.listchannels(source=l1.info['id'])['channels'])['last_update']
 
     # Get timestamps of initial updates, so we can ensure they change.