fix compilation with old gcc

Summary:
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)'

Test Plan:
builds fine now
same thing should be done to modeltest usage in kwin

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5083