Mock/Test Services folder

Must to write unit-tests for [Services](https://github.com/theiskaa/anon-app/tree/develop/lib/core/services) folder:
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Testable files:
- [Auth Service](https://github.com/theiskaa/anon-app/blob/develop/lib/core/services/auth_service.dart)
- [User Service](https://github.com/theiskaa/anon-app/blob/develop/lib/core/services/user_service.dart)