The changes on this MR overhaul the dune_python_add_test command. There was some functionality missing that could not have been added easily, so I decided to rewrite some parts of it. Now it forwards to dune_add_test and therefore uses some functionality from there, e.g. CMAKE_GUARDS, MPI_RANKS, etc.
I tested with some core modules, but this should be also tested by some of you for different setups, use-cases and custom modules.