tests: fix up some import statements caught by Python 3 I'm curious how the import checker manages to be so much more pedantic in Python 3, but not enough to bother exploring. Differential Revision: https://phab.mercurial-scm.org/D5240

tests: replace `tac` reimplementation by `sort -r` `sort -r` is better code "formatter" than `tac` since it's stable. It's also portable so we don't need to reimplement it in Python. Differential Revision: https://phab.mercurial-scm.org/D5239