test: make MiniWallet produce deterministic transactions

Summary:
MiniWallet is used in the assumeutxo functional test to produce transactions for the utxo snapshot, it needs to reliably produce the same txid for the same set of inputs and outputs.
The source of non-determinism is pad_tx, so add an option to this function to use deterministic data to pad the transaction.

Depends on D17932

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17933