glsl: Create a standalone executable for testing optimization passes.

glsl: Create a standalone executable for testing optimization passes.

This patch adds a new build artifact, glsl_test, which can be used for
testing optimization passes in isolation.

I'm hoping that we will be able to add other useful standalone tests
to this executable in the future.  Accordingly, it is built in a
modular fashion: the main() function uses its first argument to
determine which test function to invoke, removes that argument from
argv[], and then calls that function to interpret the rest of the
command line arguments and perform the test.  Currently the only test
function is "optpass", which tests optimization passes.