Add cleanstack check when activating magnetic anomaly

Summary:
As per suggestion by @dgenr8

Co-authored-by: Amaury Séchet <deadalnix@gmail.com>
Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: Added test cases in abc-magnetic-anomaly-activation.py

Reviewers: #bitcoin_abc, Mengerian, deadalnix

Reviewed By: #bitcoin_abc, Mengerian, deadalnix

Subscribers: Mengerian, teamcity, dgenr8

Differential Revision: https://reviews.bitcoinabc.org/D1647