tests: use built-in gl_Position
tests: use built-in gl_Position

This change is required for upcoming revision of glslang, which requires
location on input/outputs but which does not allow location on
built-ins.

Change-Id: I9ba8989e5ac69bf422e16d4e7ffc7d33b9f71760

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools