egl: add test for GL_EXT_EGL_image_storage

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Marge Bot <mesa/piglit!125>
Part-of: <mesa/piglit!125>
4 jobs for glEGLImageTargetTexStorageEXT in 3 minutes and 56 seconds (queued for 1 second)