knowledge_pad_ui_test: fix a random failure
This test click on the delete button which deletes by an ajax request and immediatly after open the "wait for activities" page. Sometimes the second request starts before the first is committed, so there are no activities to wait for.
When using ZServer, we had only one worker thread, so this was not visible, but with wsgi we have more than one so it happened sometimes.
Use an old jQuery trick to wait that the first request is no longer in flight.