Make sure p2p-fullblocktest.py works with the fork enabled

Summary:
This ensure it uses replay protected transactions and do not check for big blocks rejection.

Depends on D432

Test Plan: Run thet test.

Reviewers: #bitcoin_abc, CCulianu

Reviewed By: #bitcoin_abc, CCulianu

Subscribers: CCulianu

Differential Revision: https://reviews.bitcoinabc.org/D434