[WIP] Fix EZP-20877: Improve unit test code coverage of eZ\Publish\Core\SignalSlot 

- [x] test infrastructure to ease service tests
- [x] test ContentService
- [ ] test ContentTypeService
- [ ] test FieldTypeService
- [ ] test LanguageService
- [ ] test LocationService
- [ ] test ObjectStateService
- [ ] test RoleService
- [ ] test SearchService
- [ ] test SectionService
- [ ] test TrashService
- [ ] test URLAliasService
- [ ] test URLWildcardService
- [ ] test UserService
- [ ] test Repository