Fix read receipt animation (#12923)

* Fix read receipt animation

The way it was done involved remembering dom nodes and then getting
their position later when animating the receipt to its next position,
but I'm not sure how this worked since the DOM node may not neccessarily
be in the DOM anymore. Instead, just remember the bounding box coordinates.
At worst it might go weird if the window is resized but seems fine in
practice. Also, keeping references to dom nodes feels like a fast road
to memory leaks.

Fixes https://github.com/element-hq/element-web/issues/27916

* Attempt to write a test for read receipts

and fix naming

* Another test

also change a condition to make it testable