Fix EZP- 20936: update fieldtype integration tests for adding and removing field definition

This PR resolves issue https://jira.ez.no/browse/EZP-20936

Tests for removing and adding field definition to the Content's ContentType are implemented.
There are two overrides of the base tests, one for User field type (see linked issue) and other for Rating field type.

Problems fixed:
* sort key for default value in field definition needed to be set for some field types
* deleteFieldData() in KeywordStorage was not implemented
* Integer field type for empty value stored 0 instead of null

Two new issues were discovered, see linked issue for details.