Prevent phantom notifications from events not in a room's timeline (#3942)

* Test whether an event not in a room's timeline causes notification count increase

Commited separately to demonstrate test failing before.

* Don't fix up notification counts if event isn't in the room

As explained by the comment, hopefully.

* Fix other test

spec/unit/notifications.spec.ts

@@ -106,6 +106,8 @@ describe("fixNotificationCountOnDecryption", () => {
             mockClient,
         );
 
+        room.addLiveEvents([event]);
+
         THREAD_ID = event.getId()!;
         threadEvent = mkEvent({
             type: EventType.RoomMessage,