EZP-23643: Search without permissions throws UnauthorizedException

https://jira.ez.no/browse/EZP-23643

After refactoring of SearchService for performance in 5.3.3, turning on permission filtering via $filterOnUserPermissions now throws UnauthorizedException because of $contentService->loadContent() use.

The reason this was not caught was because the integration tests testing this has been disabled for a long time (since search was moved from Content Service, so pre 5.0). Found when fixing skipped tests in first commit here.