glsl: Fix optimization tests after converting src/glsl to automake.

glsl: Fix optimization tests after converting src/glsl to automake.

Commit 99476561 (automake: src/glsl and src/glsl/glcpp) changed the
build system so that src/glsl/glsl_test is not built by default.  This
inadvertently broke "make check", since the tests in
src/glsl/tests/lower_jumps (which are run by "make check") rely on
glsl_test.

This patch ensures that "make check" builds glsl_test before running
any tests.

Reviewed-by: Matt Turner <mattst88@gmail.com>