[qa] Fix p2p-fullblocktest.py for magnetic anomaly

Summary: Make sure it pad and order transactions properly.

Test Plan:

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

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2088