use SCRIPT instead of COMMAND in dune_python_add_test

dune/python/test/CMakeLists.txt

 dune_python_add_test(NAME pyrefelement
-                     COMMAND ${PYTHON_EXECUTABLE} refelement.py
+                     SCRIPT refelement.py
                      WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                      LABELS quick)
 dune_python_add_test(NAME pygeometrytype
-                     COMMAND ${PYTHON_EXECUTABLE} geometrytype.py
+                     SCRIPT geometrytype.py
                      WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                      LABELS quick)
 dune_python_add_test(NAME pytestquad
-                     COMMAND ${PYTHON_EXECUTABLE} test_quad.py
+                     SCRIPT test_quad.py
                      WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                      LABELS quick)