[qa] don't pad transactions during make_conform_to_ctor

Summary:
This is a bad idea, as if it takes effect then it will change txids and
thus can break chains of transactions. Also, it simply does not belong in
this function.

(Introduced in D2086)

Also, fixes test that relied on this behaviour.

Test Plan: test_runner.py --extended

Reviewers: jasonbcox, #bitcoin_abc, deadalnix, Fabien

Reviewed By: jasonbcox, #bitcoin_abc, deadalnix, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3094