test: Make global TRANSPORT_VERSION variable an instance variable

Currently, transport version is a global variable declared as
TRANSPORT_VERSION in v2_p2p.py. Making it an instance variable
would help in sending non empty transport version packets for
testing purposes. It might also help EncryptedP2PState be more
extensible in far future protocol upgrades.

test/functional/test_framework/v2_p2p.py

@@ -19,7 +19,6 @@ HEADER_LEN = 1
 IGNORE_BIT_POS = 7
 LENGTH_FIELD_LEN = 3
 MAX_GARBAGE_LEN = 4095
-TRANSPORT_VERSION = b''
 
 SHORTID = {
     1: b"addr",
@@ -93,6 +92,7 @@ class EncryptedP2PState:
         # has been decrypted. set to -1 if decryption hasn't been done yet.
         self.contents_len = -1
         self.found_garbage_terminator = False
+        self.transport_version = b''
 
     @staticmethod
     def v2_ecdh(priv, ellswift_theirs, ellswift_ours, initiating):
@@ -169,7 +169,7 @@ class EncryptedP2PState:
             msg_to_send += self.v2_enc_packet(decoy_content_len * b'\x00', aad=aad, ignore=True)
             aad = b''
         # Send version packet.
-        msg_to_send += self.v2_enc_packet(TRANSPORT_VERSION, aad=aad)
+        msg_to_send += self.v2_enc_packet(self.transport_version, aad=aad)
         return 64 - len(self.received_prefix), msg_to_send
 
     def authenticate_handshake(self, response):