We are testing whether "default" functionality actually works, and also testi...

We are testing whether "default" functionality actually works, and also testing whether it works from the UI. The UI option is available only in profile listing page. To view the profile listing page you need "administer profiles" permission. That is why $restricted_user needs that permission.

The purpose of $restricted_user is to check whether he is able to set default his own profile, and is unable to set default others' profile. That is the only purpose of $restricted_user.