Use the QtTestLib facilities

The tests didn't use the QtTestLib library. It was not clear if the tests
were successful or not. QtTestLib makes it very clear.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Adds test target to CMake

Configure with tests enabled:
    cmake -DBUILD_TESTS=Yes ..

To run the test suite (after building) just type:
    make test

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Merge pull request #614 from luis-pereira/qtxdg-maint

qtxdg: Use QtTestLib to perform the tests