fix compilation on old gcc

same as https://phabricator.kde.org/D5083,
gcc 4.8.5 (opensuse leap) fails building modeltest with
CMakeFiles/testHistoryModel.dir/modeltest.cpp.o: In function `ModelTest::data()':
modeltest.cpp:(.text+0x5a57): undefined reference to `bool QTest::qCompare<int, unsigned int>(int const&, unsigned int const&, char const*, char const*, char const*, int)'