Simplify testsuite driver, part 2

Summary:
- 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