Merge: simplify glslangValidator test to support different versions of the tool

As reported in #1537, different versions of glslangValidator do not output the same error messages as they do not use the same default type for undeclared variables. This PR removes an error from the glsl code (or fix it if you prefer) so different versions of the tools have the same output. There are still many more errors to check that the integration of the tool within nitc works.

Fix #1537.

Pull-Request: #1643
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>