[FEA]: Make FileCheck a required dependency for CCCL tests

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this request and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Area

General CCCL

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

CUB and libcu++ tests skip codegeneration coverage if FileCheck is not present in the system. As @bernhardmgruber rightfully [noticed](https://github.com/NVIDIA/cccl/pull/7878#discussion_r2883106762):

> if FileCheck for some reason stops being available in our test containers, our unit tests continue to pass but we turn blind.

### Describe the solution you'd like

We should require presence of FileCheck to test CCCL.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_