Add missing dependency

The wildcard target wants to redirect its argument to build/Makefile,
without declaring a dependency. This means that "make clean; make foo"
won't work.

Fix by adding the missing dependency
Add OSC "quit" command

Not very useful to map to a control on a hardware thing, but very useful
to kill dvswitch in a test suite (which will follow next)
Add test suite

This commit adds a test suite.

It uses the oscsend command and the OSC control interface to test basic
functionality of dvswitch under xvfb.