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.

/reviewed-on !1038