Fix an existing test for editing messages in threads (#3407)

While attempting to test a new change, I discovered that the test
"should allow edits to be added to thread timeline" did not actually
fail if its assertions failed. Further, those assertions were incorrect.

So this change fixes the test to create the thread, wait for it to be
initialised, and then add events to it. This simplifies the flow and
ensures the test fails if something unexpected happens.

spec/unit/event-timeline-set.spec.ts

@@ -284,19 +284,27 @@ describe("EventTimelineSet", () => {
                     sender,
                 });
 
-                jest.spyOn(client, "paginateEventTimeline").mockImplementation(async () => {
-                    thread.timelineSet.getLiveTimeline().addEvent(threadReply, { toStartOfTimeline: true });
-                    return true;
-                });
-                jest.spyOn(client, "relations").mockResolvedValue({
-                    events: [],
-                });
-
-                const thread = room.createThread(root.getId()!, root, [threadReply, editToThreadReply], false);
-                thread.once(RoomEvent.TimelineReset, () => {
-                    const lastEvent = thread.timeline.at(-1)!;
-                    expect(lastEvent.getContent().body).toBe(" * edit");
-                });
+                // Mock methods that call out to HTTP endpoints
+                jest.spyOn(client, "paginateEventTimeline").mockResolvedValue(true);
+                jest.spyOn(client, "relations").mockResolvedValue({ events: [] });
+                jest.spyOn(client, "fetchRoomEvent").mockResolvedValue({});
+
+                // Create a thread and wait for it to be initialised
+                const thread = room.createThread(root.getId()!, root, [], false);
+                await new Promise<void>((res) => thread.once(RoomEvent.TimelineReset, () => res()));
+
+                // When a message and an edit are added to the thread
+                await thread.addEvent(threadReply, false);
+                await thread.addEvent(editToThreadReply, false);
+
+                // Then both events end up in the timeline
+                const lastEvent = thread.timeline.at(-1)!;
+                const secondLastEvent = thread.timeline.at(-2)!;
+                expect(lastEvent).toBe(editToThreadReply);
+                expect(secondLastEvent).toBe(threadReply);
+
+                // And the first message has been edited
+                expect(secondLastEvent.getContent().body).toEqual("edit");
             });
         });