Adds some tests for glsl_parser_tests.py

These tests model future behavior rather than current behavior, some of
them pass, others fail.

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>
glsl_parser_test.py: Remove standalone functionality.

This removes the standalone run functionality from glsl_parser_test.
Like shader_test this was probably useful for testing while in
development, but at this point it's just dead code.

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>
glsl_parser_test.py: Simplify GLSLParserTest

This vastly simplifies the way that GLSLParserTest works. This
simplification has a couple of advantages. First, we call
PlainExecTest's __init__ rather than ExecTest's __init__, which is good
OO practice.

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>