without that check, in theory our test runner will tolerate
a non-zero return value, like throwing or failing an assert,
which is not what we want....
guess these happenend to get in by forgetting to
add this check when switching a test from PLANNED to TEST
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.
