[Piglit] Refactor Test Classes v2

[Dylan Baker]

This series refactors and simplifies the Test classes in piglit. It's
goal is to have a single abstract base class Test, which should be the
parent of most test classes. PlainExecTest (renamed PiglitTest) derives
from this class, while GLSLParserTest and ShaderTest derive from
PiglitTest. IGTTest, GTest, OGLCTest, etc descend from Test.

In the quest to do this ShaderTest and GLSSParserTest needed to be
refactored, since they relied on Test, ExecTest, and PlainExecTest being
separate from each other. They have been simplified, and in some case
optimized, but usually with a focus on better error reporting and
handling.

Finally, testBinDir was moved from framework.core to framework.exectest.
This cleans up some import dependencies between the various modules,
since most modules that need testBinDir also need Test or PiglitTest.

This is available at my github: https://github.com/dcbaker/test-refactor-v5