Generator: re-enable our van der Pol test.
Generator: re-enable our van der Pol test.

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