Investigate using Renderer.RemoveRootComponent for disposing components

[egil]

While investigating another issue I noticed the method RemoveRootComponent in the Renderer source code (https://source.dot.net/#Microsoft.AspNetCore.Components/RenderTree/Renderer.cs,260). Its in .net 6 and later, and it seems to basically do a lot of what we want wrt. to disposing of a component under test, and could allow for more targeted disposable instead of every single rendered component as now.