test: use MiniWallet for rpc_scantxoutset.py

Summary:
Backport of core#23866.

Depends on D12733.

Test Plan:
With and without wallet:

./test/functional/test_runner.py rpc_scantxoutset

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12734