[test] #262: Add verify blocks tests:

- Integration test on the overall command:
  - Use click CliRunner().invoke()
  - https://click.palletsprojects.com/en/7.x/testing/#basic-testing
    - integration tests are requesting a node
    - If the node is down, the tests are broken
- Unit tests on all the functions to test most of the major corner cases:
  - valid and a non-valid blocks signatures:
    - Kind of already in DuniterPy tests…