[FEA] Proposal to replace `cudf::test` namespace with `cudf_test` and introduce a `cudf_benchmark` namespace

There can be confusion in the test and benchmark source on where APIs are implemented and how they are expected to behave since the `cudf::test` namespace is technically also in the `cudf` namespace.
Likewise for the libcudf benchmarks, there is no namespace and so it makes it difficult to know what are local utilities vs benchmark common utilities.

I'd like to introduce the `cudf_test` namespace in place of the current `cudf::test` namespace.
And also introduce the `cudf_benchmark` namespace for benchmark common utilities.