When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.
Part of https://github.com/ooni/probe/issues/1591
While there, modernize the way in which we run tests to avoid
depending on the fake files scattered around the tree and to
use some well defined mock structures instead.
Part of https://github.com/ooni/probe/issues/1591
