Fix EZP- 20530: Fix incomplete unit & integration tests part 2

This PR resolves part of issue https://jira.ez.no/browse/EZP-20530

Incomplete tests for ```Legacy/Content/Type/ContentUpdater/Action/AddField``` are implemented. One of the tests was found superfluous.

Also while testing discovered and fixed a bug with updating external data for untranslatable field when field definition is added to the content type.

## Todos

* [ ] Implement integration tests for state of Content after adding/removing ContentType field definition.