[Piglit] Use subprocess32 for timeouts (v2)

[baker.dylan.c at gmail.com]

Changes since v1:
 - Handle forking processes properly (there is a test in the last patch
   for this which probably deserves review, I've learned a lot about how
   POSIX processes work for this series and may or may not have gotten
   it right)
 - Split the timeout code into a Mixin. This simplifies handling support
   for timeouts on systems that are either windows or don't have
   subprocess 32.
 - Better unit-testing
 - Actually terminate (and kill) processes that run too long.

This leaves much of the groundwork for timeouts on windows, but doesn't
implement them. I don't have a windows dev system, nor does my team
really care about windows. However, windows should continue to work
timeout free until such a time as someone cares to implement support.

This is available at my github:
https://github.com/dcbaker/piglit submit/subprocess32

Thomas, 

Hopefully I've addressed all of the issues you raised with the first
iteration of this series, please let me know if there are any cases I'm
not handling correctly.