[bugfix] Use the correct variable to define the default of COMMAND

cmake/modules/DuneTestMacros.cmake

@@ -186,7 +186,7 @@ function(dune_add_test)
     endif()
   endif()
   if(NOT ADDTEST_COMMAND)
-    set(ADDTEST_COMMAND ${ADDTEST_TARGET})
+    set(ADDTEST_COMMAND ${ADDTEST_NAME})
   endif()
   if(NOT ADDTEST_MPI_RANKS)
     set(ADDTEST_MPI_RANKS 1)