Test new behaviour around enumeration of custom devices

See https://github.com/KhronosGroup/OpenCL-Docs/pull/1117

A few ideas:
- Check that custom devices are included in the list reported when using `CL_DEVICE_TYPE_ALL`
- Check that `CL_DEVICE_TYPE_DEFAULT` does return the custom device when it's the only one that belongs to the platform
- Check that `CL_DEVICE_TYPE_DEFAULT` does not return a custom device if there are non-custom devices that belong to the platform
- Any others?

Start by reading the behavior described in OpenCL-Docs pull request 1117, then locate the existing OpenCL-CTS tests for device enumeration. Add coverage for the listed CL_DEVICE_TYPE_ALL and CL_DEVICE_TYPE_DEFAULT cases, and confirm the tests pass for platforms with custom and non-custom devices.

Written by the indexing model from the issue text.