Add test where units are expected to be linked when parsed.
Add test where units are expected to be linked when parsed.

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