manual: tests: Describe how to port-forward into test VMs

Once the connection is established, you can enter commands in the socat terminal

where socat is running.

## Port forwarding to NixOS test VMs {#sec-nixos-test-port-forwarding}

If your test has only a single VM, you may use e.g.

```ShellSession

$ QEMU_NET_OPTS="hostfwd=tcp:127.0.0.1:2222-127.0.0.1:22" ./result/bin/nixos-test-driver

```

to port-forward a port in the VM (here `22`) to the host machine (here port `2222`).

This naturally does not work when multiple machines are involved,

since a single port on the host cannot forward to multiple VMs.

If the test defines multiple machines, you may opt to _temporarily_ set

`virtualisation.forwardPorts` in the test definition for debugging.

## Reuse VM state {#sec-nixos-test-reuse-vm-state}

You can re-use the VM states coming from a previous run by setting the