Port test_internet.py to pytest and add new pytest-mock dependency

[NicolasHug]

This PR ports the test_internet.py tests to pytest.

It also adds the pytest-mock test-time dependency. I'll comment below for why this is probably preferable, but I'm happy to hear y'all thoughts on this.

[fmassa]

LGTM to add the extra test dependency on pytest-mock, if it's governed by pytest

[github-actions]

Hey @NicolasHug, @fmassa!

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[NicolasHug]

Thanks for the reviews!
I'll try to import this soon in fbcode to make sure there's no dependency issue, hopefully later today.

BTW, now that we rely on pytest, we could put back those tests where they used to be instead of having a dedicated file for internet tests. We could just mark them with a needs_internet mark, and just bypass those tests in fbcode, similar to what is done with the needs_cuda mark in https://github.com/pytorch/vision/pull/4025/files

[fmassa]

@NicolasHug everything should be fine in fbcode because there is no TARGET for test_internet.

I would be ok moving the tests back to where they were originally, although given that they are for now self-contained to just the download functionality, I would be fine leaving them here for now