Fix tst_QHighDpiScaling::scale

Without an override for logicalDpi() the base class implementation
would use the geometry of the screen to figure out the DPI, and end
up with ~100, which combined with a 96DPI base logical DPI would
give a wrong scale factor.

Change-Id: I68aecce44d2ee672c7b707dfe5444af8f551e961
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

tests/auto/gui/kernel/qhighdpiscaling/tst_qhighdpiscaling.cpp

@@ -47,6 +47,8 @@ public:
     QRect geometry() const override { return QRect(3840, 0, 3840, 1920); }
     QRect availableGeometry() const override { return geometry(); }
 
+    QDpi logicalDpi() const override { return logicalBaseDpi(); }
+
     int depth() const override { return 32; }
     QImage::Format format() const override { return QImage::Format_ARGB32_Premultiplied; }
 };