Add test to verify transform feedback values of few builtin variables

Builtin variables tested are gl_Layer and gl_ViewportIndex. The test
case verifies a bug fix in Intel's i965 drivers. It can be later
extended to add testing for more builtin variables.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Add test to verify the values of gl_VertexID captured by transform feedback