Fix RenderTests for PointcloudPotreeExample

[wrestledBearOnce]

This example relies upon DeltaTime(Update), however, these values are never being set.
DeltaTime(Update) is updated within RenderCanvasGameWindow.cs which only happens after the blocking method app.Run() is being called (which we do not want to do while testing).
Furthermore, the lazy loading mechanism induces the necessity to render more than the usual 3 frames until the point cloud is loaded completely.