[INFRA]: Validate relocatable installed CUB tests

Check whether CUB tests can run from an installed test tree instead of relying on build-tree paths using RAPIDS-style build-tree decoupling.

Related RAPIDS reference:
- [`rapids_test_install_relocatable`](https://github.com/rapidsai/rapids-cmake/blob/main/rapids-cmake/test/install_relocatable.cmake)

Start with the RAPIDS reference at rapids-cmake/test/install_relocatable.cmake and inspect how the CUB tests are installed and invoked. Verify whether the tests run from an installed test tree without build-tree paths; done means relocatable installed-test execution is validated or the remaining limitation is documented.

Written by the indexing model from the issue text.