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/wallet_listtransactions.py

@@ -4,22 +4,17 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test the listtransactions API."""
 from decimal import Decimal
-from io import BytesIO
 
-from test_framework.messages import COIN, CTransaction
+from test_framework.messages import (
+    COIN,
+    tx_from_hex,
+)
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_array_result,
     assert_equal,
-    hex_str_to_bytes,
 )
 
-def tx_from_hex(hexstring):
-    tx = CTransaction()
-    f = BytesIO(hex_str_to_bytes(hexstring))
-    tx.deserialize(f)
-    return tx
-
 class ListTransactionsTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 2