[PL] Add cmake files for ctests.
Fixes previous merge https://github.com/ufz/ogs/pull/1619 : the actual ctests were not added, because cmake's add_subdirectory
is not recursive.
Fixes previous merge https://github.com/ufz/ogs/pull/1619 : the actual ctests were not added, because cmake's add_subdirectory
is not recursive.