Add a trivial write only GLSL test.
Add a trivial write only GLSL test.


Former-commit-id: 7c0144cf1e95caa1d49b76543638e7e55fed6f6a
darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide