Add Unit tests for the `Crowdin` class

Currently, this class has low coverage by Unit tests - [Codecov report](https://app.codecov.io/gh/crowdin/flutter-sdk/blob/main/lib/src/crowdin.dart)

Class - [crowdin.dart](https://github.com/crowdin/flutter-sdk/blob/main/lib/src/crowdin.dart)

Tests examples - https://github.com/crowdin/flutter-sdk/tree/main/test