[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/feature_addrman.py

@@ -8,9 +8,8 @@ import os
 import re
 import struct
 
-from test_framework.messages import ser_uint256, hash256
+from test_framework.messages import ser_uint256, hash256, MAGIC_BYTES
 from test_framework.netutil import ADDRMAN_NEW_BUCKET_COUNT, ADDRMAN_TRIED_BUCKET_COUNT, ADDRMAN_BUCKET_SIZE
-from test_framework.p2p import MAGIC_BYTES
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.test_node import ErrorMatch
 from test_framework.util import assert_equal