Prefer KHR entrypoints instead of EXT for robustness tests

Prefer KHR entrypoints instead of EXT for robustness tests

When resolving function entrypoints, framework resolves EXT
entrypoints after KHR to the same pointers. There are drivers that
implement only KHR entrypoints, prefer KHR over EXT so that KHR
entrypoints will be the ones used if both extensions are supported
by the driver.

Components: OpenGL ES
VK-GL-CTS issue: 1107

Affects:
KHR-NoContext.es32.robustness.getnuniform
KHR-NoContext.es32.robustness.readnpixels

Change-Id: Iec5f7cbdd53061e105b3445f7613ee41fccc4553
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>