Interesting work, the problem I see with using standard "unit tests" as fitness functions to evolve objects (or their functions) is that the resulting program may only compute the correct output for the defined test cases:

Say for example we want a function that adds two integers, we create a test with:

    int x = 0; int y=0;
    for (int x=0,y=0;x<10;x++,y++){
        int result = sumIntegers(x,y);
        assert (result== x+y);
        writeFitness(x,y,(result-x-y)/(x+y));
    }

A correct function (with fitness == 0) can be:

   int sumIntegers(int x, int y) {
       if (x == 1 and y == 1) return 2;
       if (x == 2 and y == 2) return 4;
       if (x == 3 and y == 3) return 6;
       ...
   }

Which is not semantically what we wanted.