add initial tests for variable interfaces
add initial tests for variable interfaces

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