HelloVincenzo,
Are you using any routing in your Component.js?
Even though you are setting the rootView as App.view.xml, the routing pattern for ../index.html in which case is pattern : "" may be referring to another view.
If your App.view.xml is just to initialize the application, like below and you just want to access your application, you can set the viewName in your test to the view that is loaded from the routing pattern.
<mvc:Viewxmlns:mvc="sap.ui.core.mvc"xmlns="sap.m"displayBlock="true">
<SplitAppid="idAppControl"/>
</mvc:View>
Regards,
Armin