[qa] Add ATMP test for undersize transaction

Summary:
Part 2 of many for incorporating changes from https://github.com/bitcoinxt/bitcoinxt/pull/487
Continues T418

Depends on D1930

Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: test_runner.py abc-magnetic-anomaly-activation

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1925