add unit-tests for search classes: announcements, business types, emp…

[dev-rminds]

…loyee events, employees and fund-forms covered, added placeholder classes for remaining search types

Changes description

Developers checklist

• Autotests are passed locally
• Migration rollback works - if there is migration, check rollback
• Autotests are added:
  • bugfix - unit/feature test for this scenario if possible
  • new small/medium feature - simple feature test for positive scenarios

QA checklist

• Translations are done
• Endpoint is fast on dump - if there is new endpoint or changed query, endpoints that are using changed query - working fast on production dump, <2s