test: cover testnet4 magic in assumeutxo.py

Replace testnet3 and use MAGIC_BYTES constants.

test/functional/test_framework/messages.py

@@ -80,6 +80,7 @@ DEFAULT_MEMPOOL_EXPIRY_HOURS = 336  # hours
 MAGIC_BYTES = {
     "mainnet": b"\xf9\xbe\xb4\xd9",   # mainnet
     "testnet3": b"\x0b\x11\x09\x07",  # testnet3
+    "testnet4": b"\x1c\x16\x3f\x28",  # testnet4
     "regtest": b"\xfa\xbf\xb5\xda",   # regtest
     "signet": b"\x0a\x03\xcf\x40",    # signet
 }