[Piglit] Add support to piglit to write JUnit XML directly

[Dylan Baker]

This series adds support to piglit for multiple results backends, and
implements a JUnit XML backend as the first consumer of this feature.

JUnit is a standard XML descriptive format used by jenkins for test
reporting. Currently piglit has a script that is able to convert
piglit's json output into junit xml. There are two problems with this
script that this backend re-architecture solves. First, it skips a step
and the generation of a rather large file. Results are currently ~20MB
in size, and will only get bigger. Second, this backend is written using
the lastest python2.7 features, making it easier to port to python3.

Here at Intel we are currently deploying a jenkins framework for testing
on various generations of intel hardware in a standardized way, having
this backend is useful for us, and a project that we plan to maintain in
the long run.

This series takes a fairly conservative approach to updating the
backend, making very small changes over a large number of commits, with
the majority of the work being converting the existing JSONWriter class
into a JSONBackend class that is a child of the abstract Backend class.
This astract class is used to define the API used by backends. This is
what the first 16 patches do.

The last three patches implment the JUnit backend.