Extend test set for shared scenes (View3D.importScene)

Once QTBUG-88420 fixes Texture in combination with scenes shared between View3D items, the feature should be productized by having appropriate tests and further examples. It could be that testing will uncover further features and constructs that are disfunctional with importScene, or with importScene and multiple QQuickWindows.

Sharing a scene between multiple View3D items (demonstrated in the view3d example) is not well tested: the Lancelot tests (MultipleViews and similar) use simple scenes only. No textures, no custom materials, no effects, no light probes, no advanced features (shadows, SSAO, etc.), no dynamic changes (changes to the scene, changing texture references in materials, etc.), and no multiple windows are tested.

In reality every single feature that a scene in a View3D can have, should be tested in a shared-scene setup as well. In addition, having View3D items in different windows, while sharing the same scene, presents its own problems, as we discovered in QTBUG-88241 QTBUG-86616), and that is not tested whatsoever as of now.

• Some of these things could be tested in Lancelot,
• while some are better suited for autotests (but that implies starting to have visual autotests),
• in addition, an example to complement the view3d one could be implemented, one that also demonstrates (and tests) usage in multi-window situations.