test: use MiniWallet (P2PK mode) for feature_dersig.py

Summary:
Backport of core#22057.

Depends on D16408.

Test Plan:

./test/functional/test_runner.py feature_dersig

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16409