Add a flag to gate opcodes activation for the new HF

Summary: This per se does nothing except adding a flag to gate opcodes and the infrastructure to use this flag in tests.

Test Plan: Test involving the flag have been added for all disabled opcodes.

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1208