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