[cmake] Use labels to mark unit tests

Merged Timo Koch requested to merge feature/label-unittests into master

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)

Edited by Timo Koch

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