[Piglit] Test Cleanups v2

[Dylan Baker]

This series implements improvements for piglit's test handling, mainly
in the python layer. Generally these are just cleanups, additions of
docstrings, formatting changes, renaming long functions, and removing
deprecated code.

The big change behavioraly in this series is that that it moves the
waffle platform default selection up the python layer instead of
implicitly selecting it in the C layer. This allows means that the
python code always *knows* what the platform being selected is and can
skip tests accordingly. skipping tests in the python layer is much
faster than skipping them in the C layer, because it bypasses
initialization costs. It takes ~0.0001 seconds to skip a test in the
python layer, where it takes ~0.1 seconds to skip in the C layer.

Patches 1-9 are small cleanups
Patches 10-15 Implement the changes described above,
Patches 16-17 do some refactoring to allow more code shareing
Patch 18 uses ABC for the Test class, which is a powerful tool for
		 abstract base classes