pyln-testing: use provided outnum instead of trying to find it

Dual-funded channels won't match the old amount check. Good news is
we're already returning the outnum so we just use that.

contrib/pyln-testing/pyln/testing/utils.py

@@ -864,8 +864,7 @@ class LightningNode(object):
                 txnum = i
 
         scid = "{}x{}x{}".format(self.bitcoin.rpc.getblockcount(),
-                                 txnum,
-                                 get_tx_p2wsh_outnum(self.bitcoin, res['tx'], amount))
+                                 txnum, res['outnum'])
 
         if wait_for_active:
             self.wait_channel_active(scid)