Merge branch 'bugfix/addPythonExecToAddTest' into 'master'

use SCRIPT instead of COMMAND in dune_python_add_test

See merge request core/dune-istl!420

dune/python/test/CMakeLists.txt

 dune_python_add_test(NAME pybcrsmatrix
-                     COMMAND ${PYTHON_EXECUTABLE} bcrsmatrix.py
+                     SCRIPT bcrsmatrix.py
                      WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                      LABELS quick)