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

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