Add missing Units API to Python units test.
Add missing Units API to Python units test.

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