test: Run mempool_packages.py with MiniWallet

Summary:

This allows to run the test even when no wallet is compiled in.

Also, it is a lot nicer to read now.

Backport of core#26625.

Depends on D16431.

Test Plan:

./test/functional/test_runner.py mempool_packages

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16432