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/mempool_accept_wtxid.py

@@ -77,13 +77,13 @@ class MempoolWtxidTest(BitcoinTestFramework):
         child_one.wit.vtxinwit.append(CTxInWitness())
         child_one.wit.vtxinwit[0].scriptWitness.stack = [b'Preimage', b'\x01', witness_script]
         child_one_wtxid = child_one.getwtxid()
-        child_one_txid = child_one.rehash()
+        child_one_txid = child_one.txid_hex
 
         # Create another identical transaction with witness solving second branch
         child_two = deepcopy(child_one)
         child_two.wit.vtxinwit[0].scriptWitness.stack = [b'', witness_script]
         child_two_wtxid = child_two.getwtxid()
-        child_two_txid = child_two.rehash()
+        child_two_txid = child_two.txid_hex
 
         assert_equal(child_one_txid, child_two_txid)
         assert_not_equal(child_one_wtxid, child_two_wtxid)