[FEA] Add tests to verify headers are standalone

[jrhemstad]

Is your feature request related to a problem? Please describe.

After @PointKernel's excellent work in https://github.com/NVIDIA/cuCollections/pull/141 to ensure each cuco header is standalone, we should add some tests to ensure they stay that way.

Describe the solution you'd like

@allisonvacanti pointed out the fact that Thrust does this by simply including each header in an otherwise empty .cu file and ensuring it compiles. We should steal what they do.

See

CMake: https://github.com/NVIDIA/thrust/blob/main/cmake/ThrustHeaderTesting.cmake

Configured header: https://github.com/NVIDIA/thrust/blob/main/cmake/header_test.in