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_nulldummy.py

@@ -110,7 +110,7 @@ class NULLDUMMYTest(BitcoinTestFramework):
         self.block_submit(self.nodes[0], [test2tx], accept=True)
 
         self.log.info("Test 4: Non-NULLDUMMY base multisig transaction is invalid after activation")
-        test4tx = self.create_transaction(txid=test2tx.hash, input_details=ms_unlock_details,
+        test4tx = self.create_transaction(txid=test2tx.txid_hex, input_details=ms_unlock_details,
                                           addr=getnewdestination()[2], amount=46,
                                           privkey=self.privkey)
         test6txs = [CTransaction(test4tx)]