MDL-85565 phpunit: check for <select> before <option>

Make sure that assert_select_options() will fail when the output does
not contain a <select> and therefore does not have the given options.

question/engine/tests/helpers.php

@@ -574,6 +574,7 @@ abstract class question_testcase extends advanced_testcase {
         $dom = new DOMDocument();
         $dom->loadHTML($html);
         $selects = $dom->getElementsByTagName('select');
+        $this->assertGreaterThanOrEqual(1, $selects->count(), 'There is no <select> in the output.');
         foreach ($selects as $select) {
             if ($select->getAttribute('name') == $expectation->name) {
                 $options = $select->getElementsByTagName('option');