fix tests that break when run with QTEST_GUILESS_MAIN.
MessageComopser::UtilTests and MessageCore::StringUtilTests depend on MessageCoreSettings, that depends on ColorUtils that needs access to the palette and this is in GUI depdendend. And this means it needs QTEST_MAIN to run successfully.
CCMAIL: montel@kde.org
This reverts part of commit 5e2aafed.