cl_khr_mipmap_image: missing testing for get_image_num_mip_levels

[bashbaug]

Found while investigating https://github.com/KhronosGroup/OpenCL-Docs/issues/1241:

We do not seem to test the get_image_num_mip_levels built-in function added by cl_khr_mipmap_image.

Scenarios to test:

1. Images created with num_mip_levels equal to zero, one, and a number greater than one.
2. All image types (1D, 2D, 3D, etc.).