This adds tests for navigating the timeline using the back
and forward buttons.
It also makes sure that the buttons are disabled / enabled
at the correct times.
Bug: T133278
Change-Id: I9834dd3d3bf107557b15fdd105a946902d53b8f2
Testing the visibility / presence of things around
the dialog appears to be hard and the DOM for the dialog
remains unchanged when shown / hidden as far as I can see.
These tests test that the dialog is only shown once per
user.
And that the sequence of the dialog works in regards to the correct
buttons being present at the correct times.
Bug: T133278
Change-Id: Ia86cb69573da3e888c2897b8b50f1d2e5c61d8d4