Explain what you would like to see improved

RNTuple DAOS unit tests should be automatically run in a testing build. Specifically, -Ddaos_mock=ON should be set if testing is defined. daos_mock has no dependency on the real libdaos and is safe to run in our testing environment.

Optional: share how it could be improved

Once PR #6825 is merged, a follow-up PR will be created to address this.