cl_khr_mipmap_image: missing testing for get_image_num_mip_levels

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.).

Start by locating the existing C++ image built-in tests and the coverage for the cl_khr_mipmap_image extension. Add tests for zero, one, and greater-than-one mip levels across the listed image types, then run the relevant OpenCL Conformance Tests to confirm each scenario is covered.

Written by the indexing model from the issue text.