[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/test_framework/v2_p2p.py

@@ -220,6 +220,7 @@ class EncryptedP2PState:
             # decoy packets have contents = None. v2 handshake is complete only when version packet
             # (can be empty with contents = b"") with contents != None is received.
             if contents is not None:
+                assert contents == b""  # currently TestNode sends an empty version packet
                 self.tried_v2_handshake = True
                 return processed_length, True
             response = response[length:]