MDL-85350 questions: skip restore test for certain 3rd party plugins

The test_restore_course_with_same_stamp_questions() test assumes
that all question types using answers will also use the table
"question_answers" to store said answers. However, some 3rd party
qtypes might use custom tables and for those, the test would fail.

mod/quiz/tests/backup/repeated_restore_test.php

@@ -543,6 +543,11 @@ final class repeated_restore_test extends advanced_testcase {
                 "Cannot test edited answers for qtype_{$qtype} as it does not use answers.",
             );
         }
+        if ($DB->count_records('question_answers') === 0) {
+            $this->markTestSkipped(
+                "Cannot test edited answers for qtype_{$qtype} as it does not use the question_answers table.",
+            );
+        }
         foreach ($question2data->options->answers as $answer) {
             $answer->answer = 'New answer ' . $answer->id;
             $DB->update_record('question_answers', $answer);