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_backup.py

@@ -50,13 +50,14 @@ class WalletBackupTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 4
         self.setup_clean_chain = True
-        # nodes 1, 2,3 are spenders, let's give them a keypool=100
-        # whitelist all peers to speed up tx relay / mempool sync
+        # whitelist peers to speed up tx relay / mempool sync
+        self.noban_tx_relay = True
+        # nodes 1, 2, 3 are spenders, let's give them a keypool=100
         self.extra_args = [
-            ["-whitelist=noban@127.0.0.1", "-keypool=100"],
-            ["-whitelist=noban@127.0.0.1", "-keypool=100"],
-            ["-whitelist=noban@127.0.0.1", "-keypool=100"],
-            ["-whitelist=noban@127.0.0.1"],
+            ["-keypool=100"],
+            ["-keypool=100"],
+            ["-keypool=100"],
+            [],
         ]
         self.rpc_timeout = 120