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/p2p_segwit.py

@@ -213,9 +213,11 @@ class SegWitTest(BitcoinTestFramework):
     def set_test_params(self):
         self.setup_clean_chain = True
         self.num_nodes = 2
+        # whitelist peers to speed up tx relay / mempool sync
+        self.noban_tx_relay = True
         # This test tests SegWit both pre and post-activation, so use the normal BIP9 activation.
         self.extra_args = [
-            ["-acceptnonstdtxn=1", f"-testactivationheight=segwit@{SEGWIT_HEIGHT}", "-whitelist=noban@127.0.0.1", "-par=1"],
+            ["-acceptnonstdtxn=1", f"-testactivationheight=segwit@{SEGWIT_HEIGHT}", "-par=1"],
             ["-acceptnonstdtxn=0", f"-testactivationheight=segwit@{SEGWIT_HEIGHT}"],
         ]
         self.supports_cli = False