ui_tests: improve "field_validation" method
    
    Often when trying to check e.g. required field we pass the
    method another element as parent in order to narrow down a scope
    for validation. This way we can just pass "field" name to make the
    process easier.
    
    https://pagure.io/freeipa/issue/7546
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>