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>