Add tests for clear and setting valid and invalid items.
Add tests for clear and setting valid and invalid items.

cellml/libcellml,cellml/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml