Improve test coverage

[reichhartd]

The overall test coverage of this repository should be increased. Around 80% would be desirable.

• By migrating to the new React Native architecture we can use react-native-codegen. For the codegen we can use our TypeScript interfaces, and the transition between JavaScript and the native world would be safe.
• On the JavaScript and native side, we can write more unit test cases.
• An E2E-test-case with Detox would also be worth considering. However, it not possible to fully run the video ident process since the iOS simulator and Android emulator does not have access to the camera. Also we would need a valid transaction token.