This fixes the flaky QQuickWebEngineViewGraphics test and extends it
with a new test case.

Change-Id: I2d8a0762716cb9232fdea6473760e67ac2e7146d
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>