Don't double-add events to threads in tests (#9742)

Author: duxovni

test/test-utils/threads.ts

@@ -137,7 +137,6 @@ export const mkThread = ({
     const thread = room.createThread(rootEvent.getId(), rootEvent, events, true);
     // So that we do not have to mock the thread loading
     thread.initialEventsFetched = true;
-    thread.addEvents(events, false);
 
     return { thread, rootEvent, events };
 };