Add labels to tests

Every label creates a build_<label>_tests CMake target. Could be convenient to build e.g. all freeflow tests. We should decide on labels. Tests can have multiple labels. !1235 (merged) introduces the "unit" test label for unit tests. !1289 (merged) added "multidomain" label for multidomain tests, !1291 (merged) introduces "freeflow" and "rans", while all "rans" test are also "freeflow".