a61bf508e3 reduced the tolerance from 4 to
3 in one case, making the test more rather than less flaky on systems
with a device-pixel-ratio of 1.
Pick-to: 6.4 6.3 6.2
Change-Id: I245443f0dcb1aa40176c127025501b63f12f161b
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>