Test new behaviour around enumeration of custom devices

[kpet]

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?