[PATCH] Trivial, add AO subdevice to comedi_test

Hi guys,

I found it useful when devloping/testing comedi applications to sometimes
have an AO subdevice available in the comedi_test driver.  It's pretty
useless, as all it does is loop back AO to the AI channels, and doesn't
even support AO commands or anything, but it at least lets one use the
comedi_test driver with applications that require analog input _and_
analog output.

Let me know if this patch is useful.  It isn't very invasive and hopefully
it can make it into comedi cvs...

-Calin