yet another JS race condition fix

this should greatly reduce the number of timeouts seen in the JS tests

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

@@ -4,10 +4,20 b''
 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.set_text(
+            'import time'+
+            '\nfor x in range(3):'+
+            '\n    time.sleep(1)'
+            );
         cell.execute();
     });
 
+    this.waitFor(function(){
+        return this.evaluate(function() {
+            return $("#notification_kernel")[0].textContent.indexOf('busy') !== -1;
+        });
+    });
+
 
     // interrupt using menu item (Kernel -> Interrupt)
     this.thenClick('li#int_kernel');

IPython/html/tests/casperjs/util.js

@@ -67,6 +67,10 b' casper.wait_for_output = function (cell_num) {'
             },
             // pass parameter from the test suite js to the browser code js
             {c : cell_num});
+        },
+        function then() { },
+        function timeout() {
+            this.echo("wait_for_output timedout!");
         });
     });
 };