test: make sure all '#graph' nodes are purged after toimage tests

Author: etpinard

test/jasmine/tests/toimage_test.js

@@ -22,6 +22,7 @@ describe('Plotly.toImage', function() {
         // make sure ALL graph divs are deleted,
         // even the ones generated by Plotly.toImage
         d3.selectAll('.js-plotly-plot').remove();
+        d3.selectAll('#graph').remove();
     });
 
     it('should be attached to Plotly', function() {