Ensure CI testing for supported `libcudacxx` headers in host-only TUs

[jrhemstad]

Today, we do a limited form of testing each libcu++ header with a host-only compiler where we just include the header in an otherwise empty file and ensure that compiles.

We should have more thorough testing, so we should have a way to build/run an appropriate subset of libcu++ tests with only a host compiler to verify that this works as intended.