Add unit tests to test-cli

## Description

In order to improve the quality of `test-cli` pkg, it would be good to add some unit tests for:
- cmd/test-cli/beacon.go
- cmd/test-cli/engine.go
- cmd/test-cli/requests.go
- cmd/test-cli/validator.go

Unfortunately, for most of those scripts it is required to refactor the code in order to inject dependencies, adding mocks and improving testability