how to best separate tests from source

hello all,

I have now two separate pro files on is in src/ and the other in tests/
the app's .pro contains all the source files from src
and the test's .pro contains both tests AND source files
obviously this is not good as this is duplications and coz me headaches

What is the best way to use the main sources both for tests and for app
I want them to be separate projects with shared code base
how do I organize qt files for that ?
I saw .pri files but don;t really understad them


thanx
Eli