glsl_parser_test.py: Vastly simplify the way the config is parsed

This reduces the complexity of the code that searches for config section
and writes it into a StringIO. The new code just looks for a comment
followed by [config] to open the section and a comment followed by [end
config] to close it. It naively slurps up everything in between.
Adds some tests for glsl_parser_tests.py
glsl_parser_test.py: simplify and cleanup add_glsl_parser_dir

This removes some unnecessary code, and fixes a few pylint warnings.