Unify libcudacxx header testing with the rest of CCCL's header testing

[jrhemstad]

libcudacxx and CUB/Thrust all do per-header testing of including each public header in a TU and then compiling it.

However, this infrastructure is currently not unified. They should be.

[miscco]

I am not sure whether this is something worthwile

The cmake machinery for those tests is already unified and the differences are mostly project specific.

For example thrust has the different targets that make the setup much more complicated than what CUB / libcu++ need