test: Replace MiniWallet scan_blocks with rescan_utxos

Summary:

This avoids having to fiddle with the start and num parameters and instead use the scantxoutset RPC functionality via rescan_utxos.

Backport of core#23017.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12528