Some tests are not active

It was noted that some tests are currently hard-codedly never run or even built:

• time loop (https://i10git.cs.fau.de/walberla/walberla/-/blob/master/tests/timeloop/CMakeLists.txt)
  Fixed in !390 (merged)
• gui (https://i10git.cs.fau.de/walberla/walberla/-/blob/master/tests/gui/CMakeLists.txt)
  Cannot be tested since its GUI.
• cuda (https://i10git.cs.fau.de/walberla/walberla/-/blob/master/tests/cuda/CMakeLists.txt)
  Cannot be tested since we do not have cuda-aware MPI runners in our CI.
• core (https://i10git.cs.fau.de/walberla/walberla/-/blob/master/tests/core/CMakeLists.txt#L26)
  Fixed in !429 (merged)

Is this on purpose? We should either activate them again (ideally) or remove them completely. Having them uncommented is not a good style and is confusing.