pytest: fix false bad gossip issue in test_forward

We construct the route manually so we may not have the
channel_announcement yet.  But we can get an update from the error
packet, which can lead to:

```
2021-01-29T01:38:23.4767334Z            ValueError: 
2021-01-29T01:38:23.4767987Z            Node errors:
2021-01-29T01:38:23.4768767Z             - lightningd-1: had bad gossip messages
2021-01-29T01:38:23.4769512Z            Global errors:
2021-01-29T01:38:23.4770300Z 
2021-01-29T01:38:23.4771109Z contrib/pyln-testing/pyln/testing/fixtures.py:197: ValueError
...
2021-01-29T01:38:23.7820197Z lightningd-1: 2021-01-29T01:26:57.460Z DEBUG   gossipd: Extracted channel_update 01027217b3086ad9f3dee1fa55b94c5fd2a4b0637bec70ba727ba4151a8de5173ddc749db3502d41ab0ae164addc8fd013d2088b6a12a2f478ae0affa94d76d8845c06226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f000067000001000160136459010000060000000000000000000000010000000a000000003b023380 from onionreply 100d0000007500887217b3086ad9f3dee1fa55b94c5fd2a4b0637bec70ba727ba4151a8de5173ddc749db3502d41ab0ae164addc8fd013d2088b6a12a2f478ae0affa94d76d8845c06226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f000067000001000160136459010000060000000000000000000000010000000a000000003b023380
2021-01-29T01:38:23.7837450Z lightningd-1: 2021-01-29T01:26:57.461Z DEBUG   gossipd: Bad gossip order: WIRE_CHANNEL_UPDATE before announcement 103x1x1/0
```

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

tests/test_pay.py

@@ -1011,10 +1011,7 @@ def test_decodepay(node_factory):
 @unittest.skipIf(not DEVELOPER, "Too slow without --dev-fast-gossip")
 def test_forward(node_factory, bitcoind):
     # Connect 1 -> 2 -> 3.
-    l1, l2, l3 = node_factory.line_graph(3, fundchannel=True)
-
-    # Allow announce messages.
-    l1.bitcoin.generate_block(5)
+    l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True)
 
     # If they're at different block heights we can get spurious errors.
     sync_blockheight(bitcoind, [l1, l2, l3])