tests: unit: util: Rework test structure for C++ testing

Rework test structure to handle C++ test in a better way. Similar
approach already applied in other tests which are executed for C
and C++. Getting rid of test.inc which due to non-standard extension
was not covered by complience check.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>