Stabilize tst_SceneGraph::createTextureFromImage

The test would crash half the time on my laptop due to the scene graph
not being initialized and then returning a nullptr for the texture.
At that point texture->hasAlphaChannel() crashes.

Change-Id: I9220d9bd6abe6381752c26335f2b60fa8d2adccf
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

tests/auto/quick/scenegraph/tst_scenegraph.cpp

@@ -549,6 +549,7 @@ void tst_SceneGraph::createTextureFromImage()
     QQuickView view;
     view.show();
     QTest::qWaitForWindowExposed(&view);
+    QTRY_VERIFY(view.isSceneGraphInitialized());
 
     QScopedPointer<QSGTexture> texture(view.createTextureFromImage(image, (QQuickWindow::CreateTextureOptions) flags));
     QCOMPARE(texture->hasAlphaChannel(), expectedAlpha);