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.