test: add option to speed up tx relay/mempool sync

when `self.noban_tx_relay=True`, the following flag
`-whitelist=noban,in,out@127.0.0.1`is added to `extra_args`
to speed up tx relay/mempool sync.

test/functional/wallet_fundrawtransaction.py

@@ -45,9 +45,8 @@ class RawTransactionsTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 4
         self.setup_clean_chain = True
-        # This test isn't testing tx relay. Set whitelist on the peers for
-        # instant tx relay.
-        self.extra_args = [['-whitelist=noban@127.0.0.1']] * self.num_nodes
+        # whitelist peers to speed up tx relay / mempool sync
+        self.noban_tx_relay = True
         self.rpc_timeout = 90  # to prevent timeouts in `test_transaction_too_large`
 
     def skip_test_if_missing_module(self):