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_hd.py

@@ -23,8 +23,7 @@ class WalletHDTest(BitcoinTestFramework):
         self.num_nodes = 2
         self.extra_args = [[], ['-keypool=0']]
         # whitelist peers to speed up tx relay / mempool sync
-        for args in self.extra_args:
-            args.append("-whitelist=noban@127.0.0.1")
+        self.noban_tx_relay = True
 
         self.supports_cli = False