Fix EZP-20530 incomplete unit & integration tests part 1

This PR resolves part of issue https://jira.ez.no/browse/EZP-20530

Incomplete and missing unit tests for SearchService are implemented.
Tests for validators are marked skipped instead of incomplete (until we decide what to do with validators).

Also refactored out common service methods into a separate class.

## Todos

* [x] Resolve conflicts after merging #351