Check that only the first block of the HF needs to be > 1MB

Summary: This covers an important blind spot in the test plan.

Test Plan:

../qa/pull-tester/rpc-tests.py abc-p2p-activation.py

Reviewers: freetrader, awemany, kyuupichan, sickpig, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D205