[cmake][python][bugfix] Fix dune_python_add_test after !1130

• The argument WORKING_DIRECTORY has been accidentally removed in !1130 (merged). This causes the argument being parsed as one of the MULTI argmuents if following one of these and can break the dune_add_test call.
• The way CMAKE_GUARD is handled doesn't cover the usage described in the dune_add_test docs
• Add missing documentation for newly parsed arguments