[Piglit] replace glslparsertest standalone functionality

[Dylan Baker]

This small series replaces glsl_parser_test.py's standalone
functionality with a small utility script for running a single
glslparsertest.

The output of this file is less than ideal, however, that is becasue
core.Test does some very silly things with the output of a test. That is
a patch for another day.

I'm not set on where to put the utility files, Fabian suggested that it go
in tests/ somewhere, I like the idea of keeping little utilites
together (I'd like to move generate-glean-test.py,
piglit-print-commands.py, and piglit-merge-results.py into the utils
folder as well), and putting them in tests feels odd to me, since that
is a folder for tests, not little utilities.