Fix scroll up, down pagination test
NB: this test may not fail on Travis, although it did fail locally without a fix: #563.
Once the test has scrolled the panel to the top, to the earliest events, it should be able to forward paginate, because some degree of unpagination occurs. This does assume that unpagination will occur when scrolling to the beginning of the events and that unpagination should allow pagination again in the same direction.
Instead of checking that the first event is no longer the first event (varies due to unpagination), check instead that the most recent event can be seen when scrolling all the way down to the bottom of the TimelinePanel.
Scrolling past the bottom of content seems to have strange behaviour, which isn't a useful part of the test. So now the test will scroll down until the last event instead.
cf41155610