[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/test_framework/p2p.py

@@ -72,6 +72,7 @@ from test_framework.messages import (
     msg_wtxidrelay,
     NODE_NETWORK,
     NODE_WITNESS,
+    MAGIC_BYTES,
     sha256,
 )
 from test_framework.util import (
@@ -140,13 +141,6 @@ MESSAGEMAP = {
     b"wtxidrelay": msg_wtxidrelay,
 }
 
-MAGIC_BYTES = {
-    "mainnet": b"\xf9\xbe\xb4\xd9",   # mainnet
-    "testnet3": b"\x0b\x11\x09\x07",  # testnet3
-    "regtest": b"\xfa\xbf\xb5\xda",   # regtest
-    "signet": b"\x0a\x03\xcf\x40",    # signet
-}
-
 
 class P2PConnection(asyncio.Protocol):
     """A low-level connection object to a node's P2P interface.