Fix EZP-20978: ContentService unit tests

This PR resolves issue https://jira.ez.no/browse/EZP-20978.

Unit tests for `createContent` and `updateContent` are implemented, both methods are now 100% covered.

List of issues fixed:

* required properties `mainLanguageCode` and `contentType` in `ContentCreateStruct` were not validated
* field not passed in `ContentUpdateStruct` was not validated when updating content
* passing incomplete field data by manipulating struct properties directly was not handled
* translation value of non-translatable fields was in some cases passed to storage (not affecting Legacy Storage as it stores complete set)
* fixed legacy fixtures for Feedback form fields being empty when field definitions defining them as required - this showed up after fixing item no. 2

Also simplified the methods by extracting some logic in separate protected methods.