Fix mistake in test_importer.py.
Fix mistake in test_importer.py.

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