Make BUILD_TESTS an option

This makes
    cmake -DBUILD_TESTS=Yes ..
    cmake -DBUILD_TESTS=YES ..
    cmake -DBUILD_TESTS=On ..
    cmake -DBUILD_TESTS=ON ..
    ...
work as expected (tests will be built). If BUILD_TESTS isn't an option
only cmake -DBUILD_TESTS=Yes .. works.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Explicitly requires QtTest module

If BUILD_TESTS is On make QtTest/Qt5Test REQUIRED.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Removes the project command and cmake_minimum_required()

The tests aren't a project and shouldn't be compiled indenpendently.
Without the project() and cmake_minimum_required() it's not possible to
compile it independently.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Adds missing library (Qt5Xml) to qtxdd target

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Removes Qt4 CMake stuff

We now support Qt4 and Qt5. It's all done in the main CMakeLists.txt

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Adapts build system to support Qt4 and Qt5

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>