test: Return new_utxo from create_self_transfer in MiniWallet

Summary:

I need this for some stuff, but it also makes sense on its own to:

    unify the flow with a private _create_utxo helper
    simplify the flow by giving the caller ownership of the utxo right away

Backport of core#25445.

Depends on D12736.

Note that all the changes are not applicable to our codebase but the most important ones do.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12737