[Piglit] Use subprocess32 for test timeouts, or fake it

[baker.dylan.c at gmail.com]

This changes the way that test timeouts work to be more robust than the
current implementation. This uses the external module subprocess32 which
provides the same interface as python 3.2+, a timeout parameter for
Popen.communicate. This is much simpler than our implementation, and has
the advantage of working correctly with threads, unlike our
implementation.

Since the interface is like python 3.x, this provides us a path forward
for building piglit has a hybrid python 2.x/3.x application, as we look
toward a python 3 only future.

Thomas, Daniel:
Will this work for you? I would really like to move forward with
building a hybrid python 2/3 codebase for piglit, and this would be
really nice, since it would make timeouts work very nicely in python 3.