When you submit a binary to the App Store, you can give instructions to Apple about how to test the application. There you can provide them with test credentials.
In an older version of the iTunes Connect Developer Guide, in the section "Supply Version Information", the field "Review Notes" was described as such:
Use this field to give demo account information with full access to Apple for purposes of reviewing your application.
If your application requires specific settings, user
registrations, or account information prior to submission to the App Store, be sure to include that information in this field.
This is a text field visible only to the App Review team, so the information entered in this field will not appear on the App Store. You can also include general instructions or other relevant information about your application that you think would be useful for the review process.
(The text of the Guide has changed; the "Demo Account" field is now described in "iTunes Connect App Properties".)
You could create a test account that has access to example files.