Optimize abc-p2p-fullblocktest

Summary: Take the opportunity to feed the node a 100MB block.

Test Plan:

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

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1138