[ticket/10875] Return $query_id from sql_save

Have to return the $query_id from sql_save so that the results can be pulled

Updated cache test to do some basic sql cache testing.

PHPBB3-10875

phpBB/includes/cache/driver/file.php

@@ -385,6 +385,8 @@ class phpbb_cache_driver_file extends phpbb_cache_driver_base
 		{
 			$query_result = $query_id;
 		}
+
+		return $query_id;
 	}
 
 	/**