[FEA] Investigate removing C++ tests of detail APIs

**Is your feature request related to a problem? Please describe.**
Some C++ test files are written to test detail APIs rather than public APIs. One particular example that was brought up in https://github.com/rapidsai/cudf/pull/12888#discussion_r1132719780 is [`detail_gather_tests.cu`](https://github.com/rapidsai/cudf/blob/branch-23.04/cpp/tests/copying/detail_gather_tests.cu). Ideally we should not be testing detail APIs directly, only public APIs. In practice, we should minimize cases where we must test detail APIs directly.

**Describe the solution you'd like**
We should evaluate removing tests of detail APIs. In cases where the associated public APIs are not sufficiently tested, the detail tests should be converted to test the public APIs. In other cases where the detail tests are purely redundant they should be removed. If detail APIs are being called as part of a sequence of cudf calls in a more complex test of public APIs, those calls should be rewritten to use public APIs. The remainder should be cases where detail APIs lack an exact public analog and testing the underlying APIs is valuable. We will need to assess those tests carefully.