[qa] fix abc-p2p-compactblocks to be compatible with magnetic anomaly upgrade

Summary:
Properly use LTOR blocks, and padded transactions.

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

Test Plan: test_runner.py abc-p2p-compactblocks

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2094