protected function tearDown()
 {
     // delete all test tables
     $dbman = $this->tdb->get_manager();
     $tables = $this->tdb->get_tables(false);
     foreach ($tables as $tablename) {
         if (strpos($tablename, 'test_table') === 0) {
             $table = new xmldb_table($tablename);
             $dbman->drop_table($table);
         }
     }
     parent::tearDown();
 }