MDL-29571 questions: improve class=accesshide screen-reader hints.

This changes the hint that comes before the question text. There are
unit tests to verify the new behaviour.

(amended to fix comments)

question/type/rendererbase.php

@@ -49,6 +49,16 @@ abstract class qtype_renderer extends plugin_renderer_base {
         return $qa->get_question()->format_questiontext($qa);
     }
 
+    /**
+     * In the question output there are some class="accesshide" headers to help
+     * screen-readers. This method returns the text to use for the heading above
+     * the formulation_and_controls section.
+     * @return string to use as the heading.
+     */
+    public function formulation_heading() {
+        return get_string('questiontext', 'question');
+    }
+
     /**
      * Output hidden form fields to clear any wrong parts of the student's response.
      *