Test Wiki:Interface administrators
Interface administrators are a user group that is able to edit CSS, JSON and JavaScript pages, including site CSS/JS pages.
From 21 July 2020, for security reasons, only Stewards can grant and withdraw these rights (see Special:ListGroupRights). Formerly that group could be tested with - all bureaucrats were able to grant this permission.
Now, if you really need these rights for testing reasons, you might try to ask Stewards, but they are NOT obliged to accept the request.