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_ephemeral_dust.py

@@ -38,7 +38,7 @@ class EphemeralDustTest(BitcoinTestFramework):
         # Take value from first output
         result["tx"].vout[0].nValue -= output_value
         result["new_utxos"][0]["value"] = Decimal(result["tx"].vout[0].nValue) / COIN
-        new_txid = result["tx"].rehash()
+        new_txid = result["tx"].txid_hex
         result["txid"]  = new_txid
         result["wtxid"] = result["tx"].getwtxid()
         result["hex"] = result["tx"].serialize().hex()