Add test of comma operator in constant-expression context.

[kenrussell]

In GLSL 3.00 section 12.43 "Sequence operator and constant
expressions" this is disallowed.

Associated with http://crbug.com/1324747 .

[johnstiles-google]

Adding a test SGTM, thanks for writing the PR! Few tweaks I would suggest:

The comment "// <-- no error" is deceptive and should probably be removed. I originally put this comment here to pinpoint the place where an error ought to occur, but wasn't occurring.

The rest of the shader code after that line is just boilerplate from my shader-building tool and doesn't add value to the test.

[davychxn]

The rest of the shader code after that line is just boilerplate from my shader-building tool and doesn't add value to the test.

So, maybe this just good?

void main() {
    // Expecting compiling errors here
    int array[(2, 3)];
    out_color = vec4(1.0);
}

[johnstiles-google]

Yes, that looks better.