tst_qqmlxmlhttprequest: Use QScopedPointer.

Ensure resources are cleaned up in case of failing tests.

Change-Id: Ie27800da37beac09fec34111af276ed029dcde20
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>