test: Force named args for RPCOverloadWrapper optional args

This can avoid bugs and makes the test code easier to read, because the
order of positional args does not have to be known or assumed.

test/functional/wallet_labels.py

@@ -49,7 +49,7 @@ class WalletLabelsTest(BitcoinTestFramework):
         assert_equal(response[0]['error']['message'], "Invalid label name")
 
         for rpc_call in rpc_calls:
-            assert_raises_rpc_error(-11, "Invalid label name", *rpc_call, "*")
+            assert_raises_rpc_error(-11, "Invalid label name", *rpc_call, label="*")
 
     def run_test(self):
         # Check that there's no UTXO on the node