Add proper tests for both assignedNodes and assignedElements

[WebReflection]

This MR was blocking devs but it looked OK(ish) to me ... I don't care much about slots but I want to enable devs ... I rather merge that in but I really would like to have tests around new features added to this project.

https://github.com/WebReflection/linkedom/pull/209

This is a one-off thing that won't likely be repeated in the future.