When you submit a binary to App Store, you can give instructions to Apple about how to test the application. There you can provide them with test credentials.
In the iTunes Connect Developper Guide, in the section "Supply Version Information", the field "Review Notes" is 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.
You could create a test account that has access to example files.