tst_QGraphicsWidget: fix memleak in setStyle()

The result of QStyleFactory::create() is owned by the caller, and this
test function neglected to delete it, leaking the object.

To fix, reset() it, once created, into a unique_ptr. This is the
minimally0-invasive fix.

Amends 9bc49b0bca361646aac90cd00706588d3dcd66c9.

Pick-to: 6.8 6.5 5.15
Change-Id: I17fae174b93bd27e2cac6e5e2d2c8a4ac2de703e
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit f0a9b5b6f11dc5deaabadce0ee672530683cf0a0)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/widgets/graphicsview/qgraphicswidget/tst_qgraphicswidget.cpp

@@ -1356,9 +1356,12 @@ void tst_QGraphicsWidget::setStyle()
 
     int oldEventCounts = widget.eventCount;
 
+    std::unique_ptr<QStyle> reaper;
+
     QFETCH(QString, style);
     if (!style.isEmpty()) {
         QStyle *fstyle = QStyleFactory::create(style);
+        reaper.reset(fstyle);
         widget.setStyle(fstyle);
         QCOMPARE(widget.style(), fstyle);
     } else {