Split out from #2521

…rding to the signature

Fix test failure in `sliding-sync/sliding-sync.ts` -> `"should show the right unread notifications"`

```
Cannot call `EventTimelineSet::canContain without a `room` set. Set the room when creating the EventTimelineSet to call this method.
    at EventTimelineSet.canContain (VM4503 200:726:13)
    at EventTimelineSet.addEventToTimeline (VM4503 200:576:15)
    at EventTimelineSet.addLiveEvent (VM4503 200:527:10)
    at eval (VM4587 281:722:145)
    at Array.forEach (<anonymous>)
    at SlidingSyncSdk.purgeNotifications (VM4587 281:720:22)
    at SlidingSyncSdk.onLifecycle (VM4587 281:263:14)
    at SlidingSync.emit (VM4407 21:153:5)
    at SlidingSync.emit (VM4472 169:45:18)
    at SlidingSync.invokeLifecycleListeners (VM4588 282:378:10)
```

See https://sonarcloud.io/project/issues?resolved=false&sinceLeakPeriod=true&types=CODE_SMELL&pullRequest=2856&id=matrix-js-sdk&open=AYRElaAdNY5nTGOdJo3a