[test] Move MAGIC_BYTES to messages.py

This avoids circular dependency happening when importing MAGIC_BYTES.
Before,
	p2p.py <--import for EncryptedP2PState-- v2_p2p.py
	  |					    ^
	  |				            |
	  └---------import for MAGIC_BYTES----------┘
Now, MAGIC_BYTES are kept separately in messages.py

Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

test/functional/p2p_v2_transport.py

@@ -7,8 +7,7 @@ Test v2 transport
 """
 import socket
 
-from test_framework.messages import NODE_P2P_V2
-from test_framework.p2p import MAGIC_BYTES
+from test_framework.messages import MAGIC_BYTES, NODE_P2P_V2
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,