Add cursor input tests

Add tests for the stateful input manager and inputs.

The full list of changes will include:
- Introduce v2 API interfaces
- Introduce [compatibility layer](https://github.com/urso/beats/tree/fb-input-v2-combined/filebeat/input/v2/compat) to integrate API with existing functionality
- Introduce helpers for writing [stateless](https://github.com/urso/beats/blob/fb-input-v2-combined/filebeat/input/v2/input-stateless/stateless.go) inputs.
- Introduce helpers for writing [inputs that store a state](https://github.com/urso/beats/tree/fb-input-v2-combined/filebeat/input/v2/input-cursor) between restarts.
- Integrate new API with [existing inputs and modules](https://github.com/urso/beats/blob/fb-input-v2-combined/filebeat/beater/filebeat.go#L301) in filebeat.