fix add_dumux_test for use with Dune 2.6

add_dumux_test forwards to dune_add_test. Since Dune 2.6, this takes
only the "real" command behind COMMAND, the arguments to that command
have to follow behind CMD_ARGS.

(cherry picked from commit 13141f3f)

cmake/modules/DumuxTestMacros.cmake

@@ -55,9 +55,12 @@ macro(add_dumux_test dumux_test dumux_test_executable dumux_test_executable_sour
     # tests always require the executable to run
     set_tests_properties(${dumux_test} PROPERTIES REQUIRED_FILES ${dumux_test_executable})
   else()
+    list(GET dumux_test_args 0 dumux_test_command)
+    list(REMOVE_AT dumux_test_args 0)
     dune_add_test(NAME ${dumux_test}
                   TARGET ${dumux_test_executable}
-                  COMMAND ${dumux_test_args})
+                  COMMAND ${dumux_test_command}
+                  CMD_ARGS ${dumux_test_args})
     # tests always require the executable to run
     set_tests_properties(${dumux_test} PROPERTIES REQUIRED_FILES ${dumux_test_executable})
   endif()