[Piglit] Add tox for running framework tests with different configurations

[baker.dylan.c at gmail.com]

Disclaimer: This little series only touches the python framework tests,
and I think I'm the only one who cares about them, so if I don't see any
review in a day or so I'll just push these.

This cleans up a few tests, fixes a few others, removes some hand rolled
code in favor of code that is part of nose, and adds a test module for
the python generators located in generated_tests

The biggest change however, is the addition of tox, which orchestrates
the running of all of these tests, and does so in the various
environments that we support, which includes python 2.7 and 3.3-5 for
the generators, and python 2.7 with the optional dependencies of
backports.lzma, simplejson, and lxml or without.

This can further be modified very easily to produce junit xml output,
which can be consumed by tools like jenkins.