Add basic tests for flattening a model with a units import and a component import.
Add basic tests for flattening a model with a units import and a component import.

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