fixup! fixup! erp5_xhtml_style: add support for portal_status_level

using a css selector also matches:

    <div class="transition_message transition_message_success">Deleted.</div>

unlike the xpath selector which matches only element where class
attribute is exactly "transition_message"

bt5/erp5_ui_test_core/SkinTemplateItem/portal_skins/erp5_ui_test_core/user-extensions.js.js

@@ -130,7 +130,7 @@ Selenium.prototype.assertFloatValue = function(locator, text) {
  * Checks the portal status message.
  */
 Selenium.prototype.assertPortalStatusMessage = function(text) {
-    var psm_locator = "//div[@class='transition_message']";
+    var psm_locator = "css=div.transition_message";
     var actualValue = getText(this.page().findElement(psm_locator));
     Assert.matches(text, actualValue);
 };