Add offline mode to skip tests that require an internet connection

[Luflosi]

Closes #133.

I found a way to make make skipping the tests very elegant, requiring only @mark.online().
This comes at the expense of a little more code in conftest.py, the use of the get_closest_marker() function, which seems a bit hacky IMO and the additional file pytest.ini at the root of the project which registers the custom mark to avoid a warning.
If you think this is not worth it, I can just change this PR to skip the tests like in the APIO example.
I marked this PR as a draft to better indicate that I need your opinion on how to do this.

[ikreymer]

Thanks! I think that looks reasonable, and actually instead of pytest.ini, you can just add it to conftest.py so its all in one place. I think the following should work:

def pytest_configure(config):
    config.addinivalue_line(
        "markers", "online: marks tests as requiring an internet connection"
    )

[Luflosi]

Yes, that works, thanks. Please review.

[Luflosi]

Do you maybe have some time to review this please?