[qa] fix pad_tx to better match the desired size

Summary:
Currently, pad_tx overpads quite often. This diff makes it pad the
correct amount except in specific cases. This enables us to simplify
some other tests which need to pad blocks.

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2098