Fixed padding and tx ordering in bip68-sequence.py post-fork

Summary:
Continues T418
Depends on D1974

Co-authored-by: Dagur Valberg Johannsson <dagurval@pvv.ntnu.no>

Test Plan: ./test_runner.py bip68-sequence

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1961