ipatests: update expected webui msg for admin deletion
    
    The deletion of the admin is now forbidden (even if it is
    not the last member of the admins group) and the error
    message has changed from "admin cannot be deleted or
    disabled because it is the last member of group admins"
    to " user admin cannot be deleted/modified: privileged user".
    
    Update the expected message in the webui test.
    
    Related: https://pagure.io/freeipa/issue/8878
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>