Adding unit tests for doctrine repositories

- [x] Add helper class for repository testing
- [x] Resolve issues with mocking methods that return `QueryBuilder` ([see here for example](https://github.com/tickit-project/tickit/blob/development/src/Tickit/ProjectBundle/Tests/Manager/AttributeManagerTest.php#L104))
- [x] Write unit tests for all repositories

This PR aims to unit test doctrine repositories.