Please see instruction & login credentials below the form:
Admin User
username: admin
password: admin
Contributor Role User
username: contributor
password: contributor
Subscriber Role User
username: subscriber
password: subscriber
All files from media library are embedded on All Files Page. So, You can simply go to All Files page to test restrictions. You can see the view of admin panel by logging in with admin user but you can’t change any settings. Changing options is disabled on preview site.
More details on testing/how it works can be found here