Fix p2p-fullblocktest to pad transactions properly

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

Test Plan: test_runner.py p2p-fullblocktest

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1907