Add libfuzzer test for QQuickVectorImage

This test should be run through libfuzzer and will load a file
through QQuickVectorImage and render one frame. It sets
assumeTrustedSource (needed to load Lottie files) so it will not
necessarily work well for SVG as that disables a lot of the
security measures in place there.

Pick-to: 6.12
Task-number: QTBUG-148796
Change-Id: Ief2e2abb848c7c7eff3b2e41db434e76e0d74c36
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>