Fix corrupt thread timeline for sending events (#11260)

Events which are still in the process of being sent should *not* be included
in the `EventTimeline`. Doing so means that we will attempt to render them
twice, which makes react explode.

Fixes https://github.com/vector-im/element-web/issues/25770

test/test-utils/threads.ts

@@ -136,10 +136,6 @@ export const mkThread = ({
 
     const thread = room.createThread(rootEvent.getId()!, rootEvent, events, true);
 
-    events.forEach((event) => {
-        thread.timeline.push(event);
-    });
-
     // So that we do not have to mock the thread loading
     thread.initialEventsFetched = true;