tst_query: tolerate fractional DPR in graph position query test

The expected queriedGraphPosition was captured at a device
pixel ratio of 1. With fractional ratios the logical to
physical rounding shifts the picking ray by a sub-pixel
amount, moving the hit point by up to about 0.4 units.
Allow a tolerance below one logical pixel, about 2.7 units.

Pick-to: 6.12
Change-Id: Ie7e70178b6fdbc842f1f167c800251e242ae28e4
Reviewed-by: Sami Varanka <sami.varanka@qt.io>