Maybe I'm missing something, but it seems really stupid to me that the only tutorial provided for QtTest framework has you testing the QString class.
The typical use case for unit testing is....testing classes you wrote yourself, but there is no mention on how to reference your classes in a different project for testing in the tutorial and google has failed me as well(and I really doubt copy pasting classes is a good way to do it).
I even thumbed through 3 different Qt books with no mention of QtTest.
You can add include paths to other project directories in your .pro file like so:
Then it should be able to find the headers that you are including.
Edit: Based on comment
That's another story altogether. Undefined reference usually means you are missing a dependency. This can usually be resolved with one of two things.
The simplest is to include the missing source file:
Perhaps the better solution is to expose reusable code by compiling it as a library and linking to it. E.g. a .DLL or .LIB on Windows and .SO or .A on Linux.
Can you show us the specific errors you are getting?