fix: allow JS test to run after shutdown test

shutdown notebook test closes casper browser, here we re-instatiate it
after the test

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

@@ -26,6 +26,8 b' casper.notebook_test(function () {'
             'after shutdown: IPython.notebook.kernel.running is false ');
     });
 
+    this.thenOpen(this.get_notebook_server());
+
 //}); // end of test.begin
 });