Provide a way for the consumers of this library to create a dummy netlink connection for testing purposes

[n-peugnet]

For example, the underlying netlink package provides a nltest package, with a Dial() function that allows to create a dummy connection, and test that expected netlink messages are sent.

I see that this library already has an internal newConn() helper used for its own tests, so a way to fulfil this request would simply be to export this function, and let consumers call it together with nltest.Dial(). But maybe it would be better to put it in its own package, as netlink did it?

What do you think about this?