Test SIGHASH_FORKID activation

Summary: This ensure that transaction that use SIGHASH_FORK are invalid before the fork and become valid after the fork.

Test Plan: Added extra checks to abc-p2p-activation.py

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

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D202