Add test for checking parent when removing units from model.
Add test for checking parent when removing units from model.

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