It would be good to have an example that demonstrates the correct way to do this. There are a few details that are not currently documented, and having an example that shows the final project setup with all of these details sorted out would be really useful.

The structure I'm thinking of seems pretty typical:

root.qbs
app/
    app.qbs
lib/
    lib.qbs
tests/
    tests.qbs

Both app and tests use lib.