[qa] fix abc-p2p-fullblock test to support magnetic anomaly

Summary: Fix failures due to >100 byte tx size requirement, and CTOR.

Test Plan:

./test/functional/test_runner.py abc-p2p-fullblocktest

Reviewers: #bitcoin_abc, schancel, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, Fabien, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2086