pg16-disk-test: add mkfifo

[Omrigan]

The virtio-serial interface can be opened only once.
Consider the following scenario:

1. Process A starts writing to the serial device.
2. Process A spawns a fork, Process B. It inherits the open file descriptor.
3. Process A dies; Process B survives and preserves the file descriptor.
4. Process A is restarted but cannot open the serial device again,
   causing it to crash-loop.

To fix it, we are creating FIFO special file, which supports multiple
writers, and spawning cat to redirect it to the virtio-serial.

Part of the #939.

[sharnoff]

let's update branch to get the codespell fix, otherwise lgtm, as long as you've tested locally (as a follow-up, it'd be nice to check in CI that log forwarding works as expected).

Also, keep in mind that setting this in neondatabase/neon may not be so simple, because the control plane specifies the redirection.