Ekomark & Ekobox true unit tests

[alecandido]

Unit tests are much faster to run, and they are extremely useful to spot quickly and in a precise way if there is anything broken.

ekobox (and ekomark, but especially the other [1]) have no dedicated unit tests at the moment, so they should be written, after #79.

• add tests
• measure coverage
  https://github.com/N3PDF/eko/blob/838082430b65ccf7f91c8c8ef0966a9d75afdb76/pyproject.toml#L101-L102

[1]: ekomark is quite coupled to banana, and in general really dedicated to benchmarks, so it might be harder to unit test