Fix a cmake test linker error with Qt 5.9
I got the following error while trying to make a full Release build in openSUSE Leap 15.1:
[ 71%] Linking CXX executable ../../../bin/test_cmakefileapi
/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: CMakeFiles/test_cmakefileapi.dir/test_cmakefileapi.cpp.o: in function `TestCMakeFileApi::testConfigure()':
test_cmakefileapi.cpp:(.text._ZN16TestCMakeFileApi13testConfigureEv[_ZN16TestCMakeFileApi13testConfigureEv]+0x916): undefined reference to `bool QTest::qCompare<QString, char [4]>(QString const&, char const (&) [4], char const*, char const*, char const*, int)'
/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: test_cmakefileapi.cpp:(.text._ZN16TestCMakeFileApi13testConfigureEv[_ZN16TestCMakeFileApi13testConfigureEv]+0xdcb): undefined reference to `bool QTest::qCompare<QString, char [4]>(QString const&, char const (&) [4], char const*, char const*, char const*, int)'
collect2: error: ld returned 1 exit status