If there's a single PD Buddy Sink attached, it's pretty clear which one
we want to be testing.  As such, the testing script shouldn't require
the device to be specified if it can find exactly one Sink.  This commit
does just that, and like so, I will almost never have to figure out
which device file I need when testing a Sink again.

The recent additions to the SinkConfig class broke it, so this commit
makes it work again.

This option prints a list of connected PD Buddy Sinks.  It's handy.

To replace my old Sink testing system (GNU Screen and a text file with
the commands to send to the Sink), I wrote a Python script that tests
the Sink automatically.  The only thing I have to do (assuming the test
is successful) is press Enter a few times while watching a voltmeter's
display.  A basic testing script could be useful to other people, so I'm
including it in this Python package.

I have a vague plan to integrate this with Kaylee at some point so I
don't even have to press Enter, but that will have to wait until Kaylee
has menu support.