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/test_framework/test_framework.py

@@ -96,6 +96,7 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         """Sets test framework defaults. Do not override this method. Instead, override the set_test_params() method"""
         self.chain: str = 'regtest'
         self.setup_clean_chain: bool = False
+        self.noban_tx_relay: bool = False
         self.nodes: list[TestNode] = []
         self.extra_args = None
         self.network_thread = None
@@ -494,6 +495,10 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
             extra_confs = [[]] * num_nodes
         if extra_args is None:
             extra_args = [[]] * num_nodes
+        # Whitelist peers to speed up tx relay / mempool sync. Don't use it if testing tx relay or timing.
+        if self.noban_tx_relay:
+            for i in range(len(extra_args)):
+                extra_args[i] = extra_args[i] + ["-whitelist=noban,in,out@127.0.0.1"]
         if versions is None:
             versions = [None] * num_nodes
         if binary is None: