Make tst_QAreaSeries pixel checks device-pixel-ratio aware

checkPixels() grabs the QChartView with QWidget::grab() and samples the
result at fixed coordinates (e.g. pixel(200, 200)) to verify the area is
filled. QWidget::grab() renders at the widget's device pixel ratio, so on
a screen with a device pixel ratio greater than one the grabbed image is
larger than the logical 400x400 widget and the hard-coded coordinates no
longer sample the intended points, reading the chart background instead.

Scale the sample coordinates by the image's device pixel ratio so the
checks address the same logical points regardless of the ratio.

Pick-to: 6.12
Task-number: QTBUG-147207
Change-Id: I7f126d1ba223fe067bb0016387e4cb9023e5872e
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
Reviewed-by: Zbigniew Chyla <zbigniew.chyla@qt.io>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: SanthoshKumar Selvaraj <santhosh.kumar.selvaraj@qt.io>