Fixed tst_QWidget::updateWhileMinimized()

Fixed tst_QWidget::updateWhileMinimized()

The QBackingStore::sync() call after the first expose when the widget is
un-minimized is discarded due to Qt::WA_Mapped being false. Since there
might be backing store syncs pending on Qt::WA_Mapped being set we
should also call syncBackingStore() when we get the map event.

Change-Id: I762545dc0522a99f0b36ce9a2cd2f45894cb40a5
Reviewed-on: http://codereview.qt-project.org/5377
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>