[qa] fix pad_tx to better match the desired size
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.
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: jasonbcox, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D2098