Mark xblock initialization status using css class (in part to make testing easier)

common/static/coffee/src/xblock/core.coffee

@@ -22,6 +22,7 @@
 
     $element.trigger("xblock-initialized")
     $element.data("initialized", true)
+    $element.addClass("xblock-initialized")
     block
 
   initializeBlocks: (element) ->

common/test/acceptance/pages/xblock/acid.py

@@ -25,10 +25,7 @@ class AcidView(PageObject):
         self.context_selector = context_selector
 
     def is_browser_on_page(self):
-        return (
-            self.is_css_present('{} .acid-block'.format(self.context_selector)) and
-            self.browser.evaluate_script("$({!r}).data('initialized')".format(self.context_selector))
-        )
+        return self.is_css_present('{}.xblock-initialized .acid-block'.format(self.context_selector))
 
     def test_passed(self, test_selector):
         """