cl_khr_mipmap_image: missing testing for clEnqueueFillImage

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

We do not seem to be testing `clEnqueueFillImage` on image with mipmaps. According to the spec this should be supported:

https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#image-mipmap-access

> When the [cl_khr_mipmap_image](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#cl_khr_mipmap_image) extension is supported, the [clEnqueueReadImage](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clEnqueueReadImage), [clEnqueueWriteImage](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clEnqueueWriteImage), [clEnqueueMapImage](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clEnqueueMapImage), [clEnqueueCopyImage](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clEnqueueCopyImage), [clEnqueueCopyImageToBuffer](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clEnqueueCopyImageToBuffer), [clEnqueueCopyBufferToImage](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clEnqueueCopyBufferToImage), and [clEnqueueFillImage](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#clEnqueueFillImage) functions described above can operate on mipmapped images.

Start by locating the existing tests for clEnqueueFillImage and tests covering cl_khr_mipmap_image. Check the OpenCL 3.0 image-mipmap-access specification and verify that the test exercises clEnqueueFillImage on a mipmapped image when the extension is supported. Done means the relevant conformance coverage is present and passes.

Written by the indexing model from the issue text.