test: MiniWallet: Default fee_rate in send_self_transfer, Pass in utxo_to_spend

Summary:
Adds two new features to MiniWallet:

• The fee rate is irrelevant sometimes, so just set an arbitrary default
• The utxo to spend needs to be selected manually sometimes

This is a backport of core#19922 [1/4]
https://github.com/bitcoin/bitcoin/pull/19922/commits/faba790bd40b5a9e8849997785020790ff60571b

Depends on D10199

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10254