Add simple big block test

Summary:
The abc-p2p-fullblocktest-simple.py test generates a 2MB block
and an oversize block under EB2 conditions.

This test is intended as a starting point to derive further cases and tests,
while we look into adapting p2p-fullblocktest.py as such (it is very complex and
hard to adapt to bigger block sizes and other consensus rule changes).

Depends on D171

Test Plan: qa/pull-tester/rpc-tests.py abc-p2p-fullblocktest-simple

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D173