When setting QML2_IMPORT_PATH on tests, preserve original value too

Currently autotests are setting the QML2_IMPORT_PATH environment variable to point at the binary dir so that Purpose's QML files can be found without having to install it. However this overwrites the existing env var, which may be already set when running the tests. In KDE's CI, this means the tests can't find Kirigami, which is installed in a non-standard path.

I'm changing the CMake code so that the binary dir is prepended to the existing value of the env var, and using the correct separator so that it (hopefully?) works on Windows too.

I haven't actually tested this ^^ but hopefully it will fix the test failure at https://build.kde.org/job/Frameworks/job/purpose/job/kf5-qt5%20SUSEQt5.12/225/console