[javascript] fix test_ajax.js implementation In cubicweb.ajax.js, we maintain two lists (cw.loaded_src and cw.loaded_href) to keep track of already included CSS and JS. (see discussion about jQuery and $('foo').appendTo($head) in _loadAjaxHtmlHead.) Since all tests are executed in the "same page", those two variables need to be reinitialized before each test.