Tests: stabilize tst_QDockWidget on small screens

The test has been recently failing in CI. The problem was easy to
reproduce in a virtual machine by resizing the window of the virtual
machine small enough. This change makes sure that the requested size
hint is significantly smaller than the desktop size, to avoid the
window manager stepping in and limiting the window size.

Change-Id: Id8ce63b2b88cbed964e0330633c5d2e1dc33598c
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
(cherry picked from commit 36e6632fa3)
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

tests/auto/widgets/widgets/qdockwidget/tst_qdockwidget.cpp

@@ -735,7 +735,7 @@ void tst_QDockWidget::task169808_setFloating()
         QSize sizeHint() const
         {
             const QRect& deskRect = qApp->desktop()->availableGeometry();
-            return QSize(qMin(300, deskRect.width()), 300);
+            return QSize(qMin(300, deskRect.width() / 2), qMin(300, deskRect.height() / 2));
         }
 
         QSize minimumSizeHint() const