wait for new-notebook button to arrive

before clicking it.

Race condition was causing intermittent test failures.

IPython/html/tests/util.js

@@ -13,6 +13,7 casper.open_new_notebook = function () {
     var baseUrl = this.get_notebook_server();
     this.start(baseUrl);
     this.waitFor(this.page_loaded);
+    this.waitForSelector('#kernel-python2 a, #kernel-python3 a');
     this.thenClick('#kernel-python2 a, #kernel-python3 a');
     
     this.waitForPopup('');