MDL-77328 qformat_xml: Avoid dynamic declaration for question_answer

Declare qformat_xml::answerfiles and qformat_xml::feedbackfiles instead
of dynamically declaring answerfiles and feedbackfiles for the
question_answer instance when exporting questions and answers to XML
format.

question/format/xml/format.php

@@ -46,6 +46,12 @@
  */
 class qformat_xml extends qformat_default {
 
+    /** @var array Array of files for question answers. */
+    protected $answerfiles = [];
+
+    /** @var array Array of files for feedback to question answers. */
+    protected $feedbackfiles = [];
+
     public function provide_import() {
         return true;
     }
@@ -1203,9 +1209,9 @@ public function writequestion($question) {
                 $contextid, 'question', 'generalfeedback', $question->id);
         if (!empty($question->options->answers)) {
             foreach ($question->options->answers as $answer) {
-                $answer->answerfiles = $fs->get_area_files(
+                $this->answerfiles[$answer->id] = $fs->get_area_files(
                         $contextid, 'question', 'answer', $answer->id);
-                $answer->feedbackfiles = $fs->get_area_files(
+                $this->feedbackfiles[$answer->id] = $fs->get_area_files(
                         $contextid, 'question', 'answerfeedback', $answer->id);
             }
         }
@@ -1593,10 +1599,10 @@ public function write_answer($answer, $extra = '') {
         $output = '';
         $output .= "    <answer fraction=\"{$percent}\" {$this->format($answer->answerformat)}>\n";
         $output .= $this->writetext($answer->answer, 3);
-        $output .= $this->write_files($answer->answerfiles);
+        $output .= $this->write_files($this->answerfiles[$answer->id]);
         $output .= "      <feedback {$this->format($answer->feedbackformat)}>\n";
         $output .= $this->writetext($answer->feedback, 4);
-        $output .= $this->write_files($answer->feedbackfiles);
+        $output .= $this->write_files($this->feedbackfiles[$answer->id]);
         $output .= "      </feedback>\n";
         $output .= $extra;
         $output .= "    </answer>\n";