For testability, replace most if-based construction logic with
calls to well-tested factories living in other packages.
While there, acknowledge that a bunch of types could now be private
and make them private, modifying the code to call the public
factories allowing to construct said types instead.
Part of https://github.com/ooni/probe/issues/2121
This diff required us to move some code around, but no major
change actually happened, except better tests.
While there, I also slightly refactored ndt7's implementation and
removed the ProxyURL setting, which was actually unused.
See https://github.com/ooni/probe/issues/2121
