use wait_for_output util function

IPython/html/tests/casperjs/test_cases/check_interrupt.js

@@ -1,28 +1,23 b''
 //
 // Test kernel interrupt 
 //
 casper.notebook_test(function () {
     this.evaluate(function () {
         var cell = IPython.notebook.get_cell(0);
         cell.set_text('import time\nfor x in range(3):\n    time.sleep(1)');
         cell.execute();
     });
 
     this.thenClick('li#int_kernel');
 
-    this.waitFor(function () {
-        return this.evaluate(function get_output() {
-            var cell = IPython.notebook.get_cell(0);
-            return cell.output_area.outputs.length != 0;
-        })
-    });
+    this.wait_for_output(0);
 
     this.then(function () {
         var result = this.evaluate(function () {
             var cell = IPython.notebook.get_cell(0);
             var output = cell.output_area.outputs[0].ename;
             return output;
         })
         this.test.assertEquals(result, 'KeyboardInterrupt', 'keyboard interrupt')
     });
 });