[cmake][dune_add_test] Set test property to depend on existence of target

If the target was not found in the current state the test fails. With this
patch the ctest states "Not Run" and the name of the missing target.

cmake/modules/DuneTestMacros.cmake

@@ -302,6 +302,8 @@ function(dune_add_test)
                 COMMAND "${ACTUAL_TESTCOMMAND}" ${ACTUAL_CMD_ARGS}
                )
 
+      # Make the test depend on the existence of the target to trigger "Not Run" response
+      set_tests_properties(${ACTUAL_NAME} PROPERTIES REQUIRED_FILES ${ADDTEST_TARGET})
       # Define the number of processors (ctest will coordinate this with the -j option)
       set_tests_properties(${ACTUAL_NAME} PROPERTIES PROCESSORS ${procnum})
       # Apply the timeout (which was defaulted to 5 minutes if not specified)