Installed packages causing cmake trouble for source build.

On an Ubuntu 20.04 system when the DUNE core packages are installed (e.g. apt install libdune-grid-dev) then the cmake for a master source tool chain fails with the following error:

CMake Warning at /usr/share/dune/cmake/modules/DunePythonTestCommand.cmake:46 (message): Unparsed arguments in dune_python_add_test: This often indicates typos! Call Stack (most recent call first): dune/python/test/CMakeLists.txt:1 (dune_python_add_test)

CMake Error at /usr/share/dune/cmake/modules/DunePythonTestCommand.cmake:54 (message): dune_python_add_test: no COMMAND to execute specified! Call Stack (most recent call first): dune/python/test/CMakeLists.txt:1 (dune_python_add_test)

This means that the cmake files from the master source tree and the system get mixed up. This seem to only occur when Python bindings are enabled (-DDUNE_ENABLE_PYTHONBINDINGS=ON). However, in general it raises the question whether the mix up always occurred and it was simply not noticed.