Ensure bip68-112-113-p2p passes for pre-fork conditions

Summary:
This test starts from a clean chain, so ends up activating CTOR mid-test.

Task to fix this in the near future: https://reviews.bitcoinabc.org/T494

Test Plan: test_runner.py bip68-112-113-p2p

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2099