Rename TestRenderer to BunitRenderer

[egil]

The idea is to more clearly communicate that this is a bUnit-specific type, but not necessarily something that is used for testing (I know of multiple projects that use bUnit just for rendering Blazor components into text).

I am not too concerned about fixed documentation (aka books) with this rename, since I suspect TestRenderer is not being mentioned in either of these, as it is mostly an implementation detail.