Add KHR_lights_punctual test

[Reon90]

Could you add this one?

[cx20]

Unfortunately, I do not know the Khronos official sample for that glTF extension.
If that sample adds to glTF-Sample-Models, I would like to add it to gltf-test.

[cx20]

I discovered that there are samples below.
https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_lights_punctual/schema/examples
I would like to consider adding this sample to gltf-test.