Fix new threads not appearing. (#4009)
* Fix new threads not appearing.
We try to update the thread roots when creating a thread, but a thread
can take some time to be ready after being created so we were calling it
too soon. Add a listener for the Update event to update the thread roots
once it's ready.
Fixes https://github.com/element-hq/element-web/issues/26799
* Don't recreate the event when we update
and also add a comment to the test
* Hopefully make sonarcloud happy
