[test] make v2transport arg in addconnection mandatory and few cleanups

`TestNode::add_outbound_p2p_connection()` is the only place where
addconnection test-only RPC is used. here, we always pass the
appropriate v2transport option to addconnection RPC.

currently the v2transport option for addconnection RPC is optional.
so simply make the v2transport option mandatory instead.

test/functional/feature_anchors.py

@@ -99,7 +99,7 @@ class AnchorsTest(BitcoinTestFramework):
         self.restart_node(0, extra_args=[f"-onion={onion_conf.addr[0]}:{onion_conf.addr[1]}"])
 
         self.log.info("Add 256-bit-address block-relay-only connections to node")
-        self.nodes[0].addconnection(ONION_ADDR, 'block-relay-only')
+        self.nodes[0].addconnection(ONION_ADDR, 'block-relay-only', v2transport=False)
 
         self.log.debug("Stop node")
         with self.nodes[0].assert_debug_log([f"DumpAnchors: Flush 1 outbound block-relay-only peer addresses to anchors.dat"]):