[test][python] allow to overwrite the interpreter for python test command

When using dune_python_add_test the SCRIPT is executed by default with
the detected python interpreter. The optional INTERPRETER parameter allows
to overwrite this setting, e.g. to pass additional command line arguments
or to execute the tests using pytest.

cmake/modules/DunePythonTestCommand.cmake

@@ -37,7 +37,7 @@ function(dune_python_add_test)
   include(CMakeParseArguments)
   set(OPTION)
   set(SINGLE WORKING_DIRECTORY NAME)
-  set(MULTI SCRIPT COMMAND LABELS)
+  set(MULTI SCRIPT COMMAND LABELS INTERPRETER)
   # set(MULTI COMMAND LABELS)
   cmake_parse_arguments(PYTEST "${OPTION}" "${SINGLE}" "${MULTI}" ${ARGN})
   if(PYTEST_COMMAND)
@@ -48,6 +48,9 @@ function(dune_python_add_test)
   endif()
 
   # Apply defaults
+  if(NOT PYTEST_INTERPRETER)
+    set(PYTEST_INTERPRETER ${Python3_EXECUTABLE})
+  endif()
   if(NOT PYTEST_WORKING_DIRECTORY)
     set(PYTEST_WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
   endif()
@@ -65,7 +68,7 @@ function(dune_python_add_test)
 
   # Actually run the command
   add_custom_target(target_${PYTEST_NAME}
-                    COMMAND ${Python3_EXECUTABLE} ${PYTEST_SCRIPT}
+                    COMMAND ${PYTEST_INTERPRETER} ${PYTEST_SCRIPT}
                     WORKING_DIRECTORY ${PYTEST_WORKING_DIRECTORY})
 
   # Build this during make test_python
@@ -75,7 +78,7 @@ function(dune_python_add_test)
   dune_declare_test_label(LABELS ${PYTEST_LABELS})
   # Also build this during ctest
   _add_test(NAME ${PYTEST_NAME}
-            COMMAND ${Python3_EXECUTABLE} ${PYTEST_SCRIPT}
+            COMMAND ${PYTEST_INTERPRETER} ${PYTEST_SCRIPT}
             WORKING_DIRECTORY ${PYTEST_WORKING_DIRECTORY}
             )
   # Set the labels on the test