[cmake] Use labels to mark unit tests
This adds labels to unit tests. This automatically creates a target
build_unit_tests or if any other label is used build_<label>_tests.
Unfortunately this feature is not included in the Dune 2.6 release branch.
Depends on https://gitlab.dune-project.org/core/dune-common/merge_requests/575 to be merged (backport to dune 2.6)