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/util.py

@@ -19,7 +19,6 @@ import unittest
 
 from . import coverage
 from .authproxy import AuthServiceProxy, JSONRPCException
-from io import BytesIO
 from typing import Callable, Optional
 
 logger = logging.getLogger("TestFramework.utils")
@@ -528,7 +527,7 @@ def gen_return_txouts():
 def create_lots_of_big_transactions(node, txouts, utxos, num, fee):
     addr = node.getnewaddress()
     txids = []
-    from .messages import CTransaction
+    from .messages import tx_from_hex
     for _ in range(num):
         t = utxos.pop()
         inputs = [{"txid": t["txid"], "vout": t["vout"]}]
@@ -536,8 +535,7 @@ def create_lots_of_big_transactions(node, txouts, utxos, num, fee):
         change = t['amount'] - fee
         outputs[addr] = satoshi_round(change)
         rawtx = node.createrawtransaction(inputs, outputs)
-        tx = CTransaction()
-        tx.deserialize(BytesIO(hex_str_to_bytes(rawtx)))
+        tx = tx_from_hex(rawtx)
         for txout in txouts:
             tx.vout.append(txout)
         newtx = tx.serialize().hex()