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_balance.py

@@ -53,15 +53,14 @@ class WalletTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 2
         self.setup_clean_chain = True
+        # whitelist peers to speed up tx relay / mempool sync
+        self.noban_tx_relay = True
         self.extra_args = [
             # Limit mempool descendants as a hack to have wallet txs rejected from the mempool.
             # Set walletrejectlongchains=0 so the wallet still creates the transactions.
             ['-limitdescendantcount=3', '-walletrejectlongchains=0'],
             [],
         ]
-        # whitelist peers to speed up tx relay / mempool sync
-        for args in self.extra_args:
-            args.append("-whitelist=noban@127.0.0.1")
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()