arb_enhanced_layouts: add xfb_offset overlap test

From the GL_ARB_enhanced_layouts spec:

  " No aliasing in output buffers is allowed: It is a compile-time or
    link-time error to specify variables with overlapping transform
    feedback offsets."

v2: added forgotten check_link.

Cc: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

tests/spec/arb_enhanced_layouts/compiler/transform-feedback-layout-qualifiers/xfb_offset/invalid-overlap.vert

+// [config]
+// expect_result: fail
+// glsl_version: 1.40
+// check_link: true
+// require_extensions: GL_ARB_enhanced_layouts
+// [end config]
+//
+// From the GL_ARB_enhanced_layouts spec:
+//
+//   " No aliasing in output buffers is allowed: It is a compile-time
+//     or link-time error to specify variables with overlapping
+//     transform feedback offsets."
+
+#version 140
+#extension GL_ARB_enhanced_layouts: require
+
+layout(xfb_offset = 0) out vec4 a;
+layout(xfb_offset = 0) out vec4 b;
+
+void main()
+{
+  a = vec4(1.0);
+  b = vec4(0.0);
+}