Migrate abc-mempool-accept-txn.py off of ComparisonTestFramework

Summary: Progress towards T669

Test Plan: test_runner.py abc-mempool-accept-txn

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3890