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

# Tasks

- [x] test infrastructure to ease service tests
- [x] test ContentService
- [x] test ContentTypeService
- [x] test FieldTypeService
- [x] test LanguageService
- [x] test LocationService
- [x] test ObjectStateService
- [x] test RoleService
- [x] test SearchService
- [x] test SectionService
- [x] test TrashService
- [x] test URLAliasService
- [x] test URLWildcardService
- [x] test UserService
- [x] test Repository

# Tests

See https://gist.github.com/dpobel/5620758 for the full results of `$ phpunit --coverage-text eZ/Publish/Core/SignalSlot/Tests/`
Short: services are 100% covered, the repository is at ~95%

the signal slot services are 100% covered, the signal slot repository is about 95% covered.