`glslangValidator` will only return [the codes 0..6](https://github.com/KhronosGroup/glslang/blob/b481744aea1ecf52ee4591afaa0f5e270b9d1636/StandAlone/StandAlone.cpp#L117-L125). Fail the test if anything else is returned (like due to the exe crashing).

Also set `LD_LIBRARY_PATH` to contain the `lib` directory before calling glslang.