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_listsinceblock.py

@@ -26,7 +26,7 @@ class ListSinceBlockTest(BitcoinTestFramework):
         self.num_nodes = 4
         self.setup_clean_chain = True
         # whitelist peers to speed up tx relay / mempool sync
-        self.extra_args = [["-whitelist=noban@127.0.0.1"]] * self.num_nodes
+        self.noban_tx_relay = True
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()