Migrated abc-mempool-coherence-on-activations.py off of ComparisonTestFramework

Summary: Progress towards T669

Test Plan: test_runner.py abc-mempool-coherence-on-activations

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Maniphest Tasks: T669

Differential Revision: https://reviews.bitcoinabc.org/D3991