pytest: fix flaky 'Bad gossip' error in test_block_backfill

Sometimes the l3 seeker asks for scids, and the reply contains the
channel which is then closed by the time it checks, so it considers
the updates bad gossip.

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

tests/test_db.py

@@ -79,6 +79,10 @@ def test_block_backfill(node_factory, bitcoind, chainparams):
     # Make sure we also have the needle we added to the haystack above
     assert(31337 in [r['satoshis'] for r in l3.db_query("SELECT satoshis FROM utxoset")])
 
+    # Make sure that l3 doesn't ask for more gossip and get a reply about
+    # the closed channel (hence Bad gossip msgs in log).
+    l3.daemon.wait_for_log('seeker: state = NORMAL')
+
     # Now close the channel and make sure `l3` cleans up correctly:
     txid = l1.rpc.close(l2.info['id'])['txid']
     bitcoind.generate_block(1, wait_for_mempool=txid)