Remove duplication in test-code.

[1uc]

There's a trait to hide the difference between attributes and datasets. Once: https://github.com/BlueBrain/HighFive/blob/acf0fea662e69ff8928f055b11183c7a8811fa0e/tests/unit/tests_high_five_base.cpp#L2078

Twice: https://github.com/BlueBrain/HighFive/blob/acf0fea662e69ff8928f055b11183c7a8811fa0e/tests/unit/tests_high_five_base.cpp#L2949

Place them in a separate header and remove one of the copies.