[llvm] [libc++] Move test tools CI job back to k8s runner sets (PR #226564)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

I guess this LGTM if you're okay with this, but the `llvm-premerge-libcxx-runners` should definitely be online.

https://github.com/llvm/llvm-project/pull/226564