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/p2p_eviction.py

@@ -20,7 +20,11 @@ from test_framework.blocktools import (
     create_block,
     create_coinbase,
 )
-from test_framework.messages import CTransaction, FromHex, msg_pong, msg_tx
+from test_framework.messages import (
+    msg_pong,
+    msg_tx,
+    tx_from_hex,
+)
 from test_framework.p2p import P2PDataStore, P2PInterface
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal
@@ -89,7 +93,7 @@ class P2PEvict(BitcoinTestFramework):
                     'scriptPubKey': prevtx['vout'][0]['scriptPubKey']['hex'],
                 }],
             )['hex']
-            txpeer.send_message(msg_tx(FromHex(CTransaction(), sigtx)))
+            txpeer.send_message(msg_tx(tx_from_hex(sigtx)))
             protected_peers.add(current_peer)
 
         self.log.info("Create 8 peers and protect them from eviction by having faster pings")