[Piglit] Refactor Test classes

[Dylan Baker]

This series Reworks how python tests classes are strucutred, with a goal
of simplifying the code. 

In glsl_parser_test and shader_test this means replacing classes with
funcions. The reason for this is pretty simple, the only thing that was
being done in the classes was parsing the test file to fill in data for
the parent class. Replacing this with a function makes the code cleaner,
less complex, and means that we have fewer children to deal with.

Patches 1-3 Rework the shader_test module.
Patches 4-9 Rework the glsl_parer_test module
Patches 10-12 Refactor Test, ExecTest, and PlainExec test to create a
              single abstract base class that all children inherit from.