MDL-82939 core_sms: Add class to the SMS table

This makes it possible to reference in behat table steps.

sms/classes/table/sms_gateway_table.php

@@ -49,6 +49,8 @@ public function __construct() {
         $this->setup_column_configuration();
         $this->set_filterset(new sms_gateway_table_filterset());
         $this->setup();
+        $tableclasses = $this->attributes['class'] . ' ' . $this->get_table_id();
+        $this->set_attribute('class', $tableclasses);
     }
 
     #[\Override]