MDL-53304 question behaviour: don't show useless Check buttons

Previously, the Check button was often shown disabled when it
could not be used (e.g. when the question was finished, or when an
interactive question was in the try-again state). Eventually we
realised it was better usability to hide it in these cases.

Note that when a teacher reviews an in-progress quiz attempt, they will
see a disabled Check button if the student doing the quiz can see the
button.

question/behaviour/rendererbase.php

@@ -208,6 +208,9 @@ abstract class qbehaviour_renderer extends plugin_renderer_base {
      * @return string HTML fragment.
      */
     protected function submit_button(question_attempt $qa, question_display_options $options) {
+        if (!$qa->get_state()->is_active()) {
+            return '';
+        }
         $attributes = array(
             'type' => 'submit',
             'id' => $qa->get_behaviour_field_name('submit'),