Add support in FormTester#submit(String|Component) for Ajax submitters

At the moment if an application wants to submit a form with AjaxButton it has to use:
formTester.setValue(...); // one or more times
wicketTester.executeAjaxEvent("path:to:AjaxButton", "click");

I think formTester.submit(String|Component) should check whether the component is AjaxButton|AjaxSubmitLink and do wicketTester.executeAjaxEvent(ajaxSubmitter, "click") internally.