add tests for required feature couplings

[bashbaug]

See comment on https://github.com/KhronosGroup/OpenCL-Docs/pull/738.

We have several OpenCL C features that require other OpenCL C features, for example:

OpenCL C compilers that define the feature macro __opencl_c_device_enqueue must also define the feature macro __opencl_c_generic_address_space.

We should have a test (probably in the compilers suite) to verify the required couplings.