test: introduce tx_from_hex helper for tx deserialization

`FromHex` is mostly used for transactions, so we introduce a
shortcut `tx_from_hex` for `FromHex(CTransaction, hex_str)`.

test/functional/test_framework/messages.py

@@ -195,10 +195,17 @@ def FromHex(obj, hex_string):
     obj.deserialize(BytesIO(hex_str_to_bytes(hex_string)))
     return obj
 
+
 # Convert a binary-serializable object to hex (eg for submission via RPC)
 def ToHex(obj):
     return obj.serialize().hex()
 
+
+def tx_from_hex(hex_string):
+    """Deserialize from hex string to a transaction object"""
+    return FromHex(CTransaction(), hex_string)
+
+
 # Objects that map to bitcoind objects, which can be serialized/deserialized