Continued from #6536

The current implementations of `ChildNode::before` and
`ChildNode::after` do not match the WHATWG spec. This commit updates the
implementations to match the spec.

Our current implementation of `ChildNode::after` passes all the WPT
tests. So I made sure to add a regression test that failed with the
current implementation. There are a few other unit tests I added
to exhaust other corner cases I encountered.