Remove manual byte editing in wallet_tx_clone func test

Summary:
Replace byte editing in a raw hexadecimal transaction by a more readable element swap in a CTransaction.vout list

This is a backport of Core PR15397

Test Plan: ninja && test/functional/test_runner.py wallet_txn*

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8362