Update annoying TimelinePanel test
* Actually test what we were supposed to be testing (viz, that we can paginate
back down after we hit the top of the room)
* Make the cap on the number of events we show in the timeline a configurable
property, so that we can set it in the test
* Use a smaller cap in the test, so that we have to do less paginating to hit
the cap, to make the test run quicker.
* add some more logging so that we can see how far it's got if it gets stuck.
