tst_QParallelAnimationGroup: fix memleak

Setting the parent of a QObject to nullptr means the ex-parent no
longer owns and deletes the object as its child, leaking it.

Fix by creating a scope-guard to defer deletion until the tests have
run.

This is simpler than the alternatives:

Putting it into unique_ptr would require a new variable name, or a
larger refactoring of the function, because the `test` variable is
being re-used for many different objects in the course of the
function, most of which should not be deleted.

Using QAutoPointer would drag in QtWidgetsPrivate, and the class is
probably not available in all active branches.

Finally, deleteLater() would require reliably returning to the event
loop, which may not happen if the test is run in isolation.

Bug exists since the dawn of the public history, and QScopeGuard is
available in all active branches.

Pick-to: 6.6 6.5 6.2 5.15
Change-Id: Ib4fcb44b0b68d4ccbcf5af144a18ffb378a72213
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

tests/auto/corelib/animation/qparallelanimationgroup/tst_qparallelanimationgroup.cpp

@@ -7,6 +7,7 @@
 #include <QSignalSpy>
 
 #include <QtCore/qparallelanimationgroup.h>
+#include <QtCore/qscopeguard.h>
 
 Q_DECLARE_METATYPE(QAbstractAnimation::State)
 
@@ -928,6 +929,7 @@ void tst_QParallelAnimationGroup::autoAdd()
 
     test = static_cast<TestAnimation2*>(group.animationAt(0));
     test->setParent(0);    // remove the last one (with duration = 250)
+    const auto deleteParentlessObject = qScopeGuard([test] { delete test; });
     QCOMPARE(test->group(), static_cast<QAnimationGroup*>(0));
     QCOMPARE(group.duration(), 0);
 }