ipatests: add integration test for "Read radius servers" perm
    
    Add a new integration test for the following scenario:
    - create a user with the "User Administrator" role
    - as this user, create a user with a --radius=<radius_proxy_server>
    
    This scenario was previously failing because ipa user-add --radius
    requires read access to the radius server entries, and there was no
    permission granting this access.
    
    Related to https://pagure.io/freeipa/issue/7570
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>