pytest: fix race in test_renepay where we didn't wait for all channels.

get_channel_scid() returns the *first* channel, and we want all the channels.
So they may not actually be all visible in gossip.

```
    @unittest.skipIf(TEST_NETWORK == 'liquid-regtest', "broken for some reason")
    def test_hardmpp2(node_factory, bitcoind):
        """Credits to @daywalker90 for this test case."""
        opts = {"disable-mpp": None, "fee-base": 0, "fee-per-satoshi": 10}
        l1, l2, l3 = node_factory.get_nodes(3, opts=opts)
        start_channels(
            [
                (l1, l2, 100_000),
                (l1, l2, 200_000),
                (l1, l2, 300_000),
                (l1, l2, 400_000),
                (l2, l3, 100_000),
                (l2, l3, 200_000),
                (l2, l3, 300_000),
                (l2, l3, 600_000),
            ]
        )
        # FIXME: changing the last channel from 600k to 400k will fail the test due
        # to l2 not accepting to forward any amount above 200k with error:
        # CHANNEL_ERR_CHANNEL_CAPACITY_EXCEEDED, still investigating
        inv = l3.rpc.invoice("800000sat", "inv", "description")
>       l1.rpc.call("renepay", {"invstring": inv["bolt11"]})

tests/test_renepay.py:797: 
...
        elif "error" in resp:
>           raise RpcError(method, payload, resp['error'])
E           pyln.client.lightning.RpcError: RPC call failed: method: renepay, payload: {'invstring': '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'}, error: {'code': 205, 'message': "minflow couldn't find a feasible flow: failed to find a feasible flow: find_admissible_path failed"}
```

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

Author: rustyrussell

tests/test_renepay.py

@@ -321,17 +321,19 @@ def start_channels(connections):
     bitcoind.generate_block(1)
     sync_blockheight(bitcoind, nodes)
     txids = []
+    channel_ids = []
     for src, dst, fundamount in connections:
-        txids.append(
-            src.rpc.fundchannel(dst.info["id"], fundamount, announce=True)["txid"]
-        )
+        fc = src.rpc.fundchannel(dst.info["id"], fundamount, announce=True)
+        txids.append(fc["txid"])
+        channel_ids.append(fc['channel_id'])
 
-    # Confirm all channels and wait for them to become usable
+    # Confirm all channels
     bitcoind.generate_block(1, wait_for_mempool=txids)
     scids = []
-    for src, dst, fundamount in connections:
-        wait_for(lambda: src.channel_state(dst) == "CHANNELD_NORMAL")
-        scid = src.get_channel_scid(dst)
+    for conn, channel_id in zip(connections, channel_ids):
+        src = conn[0]
+        wait_for(lambda: ['short_channel_id' in c for c in src.rpc.listpeerchannels()['channels'] if c['channel_id'] == channel_id] == [True])
+        scid = only_one([c['short_channel_id'] for c in src.rpc.listpeerchannels()['channels'] if c['channel_id'] == channel_id])
         scids.append(scid)
 
     # Make sure they have all seen block so they don't complain about