tests: add device descriptor test

This is just an initial skeleton of
how the tests could look like.
It's proving hard to think of a good
way to encapsulate the tests. Every
test will need a different set of files
from the project compiled, as some
files should be stubs instead. Additionally,
those stubs could have some commonalities.
It should for example be possible to initialize
the usb device from the tests. It would be good
to implement at least some basic handling
of the usb registers as well. - ie. if you
enable an endpoint and put in data through fifo,
the endpoint should be disabled. It should be
possible to send data from the host to the device
as well - reading from fifo. And stuff like that...