test: use MiniWallet for feature_csv_activation.py

Summary:
Backport of core#21900.

Depends on D16404.

This diverges quite a bit from core source material due to the pushonly rule, but an effort has been made to keep it as close as possible.

Test Plan:

./test/functional/test_runner.py feature_csv_activation

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16405