QmlEngine::offlineStoragePath() should point to temporary folder when executing tests

Running QML tests currently leaves a lot of folders QML/OfflineStorage/Databases and SqLite db files under $HOME/.local/share or AppData\Local .

/QtProject/tst_qdeclarativeengine/QML/OfflineStorage
./QtProject/tst_qqmlengine/QML/OfflineStorage
./tst_examples/QML/OfflineStorage
./Qt Project/samegame/QML/OfflineStorage
./snake/QML/OfflineStorage
./data/QML/OfflineStorage
./sqllocalstorage/QML/OfflineStorage

This is undesirable as it clutters the HOME folder and might have an influence on parallel or repeatedly executed tests.