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.
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.