Simplify testsuite driver, part 2
- Avoid import *; this helps tools such as pyflakes.
The last occurrence in runtests.py is not easy to remove
as it's used by .T files.
- Use False/True instead of 0/1.
Test Plan: validate
Reviewers: bgamari, thomie, simonmar
Reviewed By: thomie
Subscribers: rwbarton, carter
Differential Revision: https://phabricator.haskell.org/D5062