add test for use case: model with two variables, one with initial value set by variable reference to the other
add test for use case: model with two variables, one with initial value set by variable reference to the other

libcellml/buildbot-testing,hsorby/libcellml,hsorby/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,hsorby/libcellml,MichaelClerx/libcellml,cellml/libcellml,hsorby/libcellml,libcellml/buildbot-testing,nickerso/libcellml,nickerso/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml