Simulate minor edit in _rtselser end point

This appends a comment node to the body to force selser to drop into the body.
This exposes selser issues when serializing top-level content elements.

TODO:
- Do something like this in rt testing
- (maybe) do some more advanced edits

Bug: 56601
Change-Id: I8b1ef27b92e0be058aedfb93999c3aa7b48ddfbb