131 error on POST on 5150 machines

[dbalsom]

5150 machines produce a '131' error on POST.

MZD says about error 131: https://www.minuszerodegrees.net/5150/post/5150%20-%20POST%20-%20Some%20errors.htm

Failure of cassette port wrap-around (loopback) test. Wrap-around diagram at here.

I don't emulate the cassette interface so this is mostly expected; but I should be able to implement something basic enough to make the BIOS happy.