tst_qqmlpreview::fps() fails on MinGW / Windows 7 with newer qtbase

With qtbase commit 139240ac671953f0763308f235d8b868c1efe1b1, the test mentioned in the summary fails.

We need to identify which change in qtbase caused the failure and decide how to either fix it or potentially revert.

Examples of failures:

https://codereview.qt-project.org/c/qt/qtdeclarative/+/274467

https://testresults.qt.io/logs/qt/qtdeclarative/4813276e8003d85a8b7000ef0dccb6f7b9010c78/WindowsWindows_7x86WindowsWindows_7x86Mingw73qtci-windows-7-x86-3-8f171c/537415a5145f37257de94a5c1e1256dcd305a523/test_1568908073/log.txt.gz

https://testresults.qt.io/coin/integration/qt/qtdeclarative/tasks/1568907906