Doxygen will only process files with a @file documentation comment.
Up to now, none of our test code has such a comment, preventing the
cross-links to unit tests from working.
This is unfortunate, since unit tests, and even the code comments there,
can be considered as the most useful form of technical documentation.
Thus I'll start an initiative to fill in those missing comments automatically
the buildsystem will now pick up and link
all test cases according to the layer, e.g.
backend tests will automatically be linked
against the backend + library solely.
