Add test to show removal of expired weak variables.
Add test to show removal of expired weak variables.

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