Fixes#10979
Testing code that listens on a specific port has created a long battle with flakes. Previous attempts to deal with this include opening a listener on a port chosen by the OS, then closing the listener, noting the port and starting the test with that port.
This still flakes, notably in macOS which has a proclivity to reuse ports quickly.
Instead of fighting with the chaos that is an OS networking stack, this PR fakes the host networking in tests.
I've taken a small step here, only faking out the Listen() calls that port-forward makes, but I think over time we should be transitioning all networking the CLI does to an abstract interface so we can fake it. This allows us to run in parallel without flakes and
presents an opportunity to test error paths as well.
Spike Curtis