[Piglit] Final set of exectest re-factors

[Dylan Baker]

This series is the final set of patches in my extensive exectest cleanup
effort. It's also by far the most aggressive and introduces the most
visible changes.

The biggest change is obviously that this series merges exectest,
gleantest, shadertest, and glsl_parser_test into a single module called
exectest. I chose to leave it as exectest since we use the folder tests
for framework unit tests. I think that putting all of the 'native' test
classes and helpers in a single module makes sense for a couple of
reasons.
1) gleantest, shadertest, and glsl_parser_test all needed to import Test
   or PiglitTest from exectest.
2) All of the GL profiles (except sanity) require imports from all of
   the listed modules.
By putting all of these classes in one module we have one namespace to
deal with, which is nice. It also removes boilerplate and allows us to
stop exposing as many internals of the exectest module so that other
modules can use them.

The other major change is to mark more of our methods as private,
currently we provide pretty much every method as public, but there are
numerous methods that are not intended to be called externally and they
should be marked as such.

This series also continues the work of modernizing and optimizing the
test classes. It cleans up some remaining PEP8 problems, especially in
GleanTest, uses the abc module for abstract classes, fixes more style
problems, moves code out of base classes into more appropriate classes,
and updates docstrings.

This is available at my github: https://github.com/dcbaker/test-cleanup-2