test: rename CTransaction .rehash()/.hash -> .txid_hex for consistency

Note that we unfortunately can't use a scripted diff here, as the same
property and method name is also used for `CBlockHeader`/`CBlock` instances.

test/functional/feature_fee_estimation.py

@@ -61,7 +61,7 @@ def small_txpuzzle_randfee(
     tx.vout[0].nValue = int((total_in - amount - fee) * COIN)
     tx.vout.append(deepcopy(tx.vout[0]))
     tx.vout[1].nValue = int(amount * COIN)
-    txid = tx.hash
+    txid = tx.txid_hex
     tx_hex = tx.serialize().hex()
 
     batch_reqs.append(from_node.sendrawtransaction.get_request(hexstring=tx_hex, maxfeerate=0))