Merge #9739: Fix BIP68 activation test

Summary:
PR9739 backport. Seems we missed it. I noticed the same logic bug
(the assert not before_activation is useless because it just gets
covered up by the except: suppression!).

f5aba8a Move tx version 2 standardness check to after bip68 activation (John Newbery)
99c0e81 Fix BIP68 activation test (John Newbery)

Test Plan:

test_runner.py feature_bip68_sequence

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5206