pytest: test_onchaind_replay needs to wait for tx broadcast.

Waiting for 'Disabling channel' is not enough, since it's async to the
actual tx broadcast: I caught a case where the unilateral close wasn't
in the block, and so failed.

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

tests/test_lightningd.py

@@ -1730,7 +1730,7 @@ class LightningDTests(BaseLightningDTests):
             'channel': '1:1:1'
         }
         l1.rpc.sendpay(to_json([routestep]), rhash)
-        l1.daemon.wait_for_log(r'Disabling channel')
+        l1.daemon.wait_for_log('sendrawtx exit 0')
         bitcoind.rpc.generate(1)
 
         # Wait for nodes to notice the failure, this seach needle is after the