Potential fix to BurnDownApp flakiness

BurnDownAppSpec was waiting for the Ajax callback to fire, but then inspecting the addSpy callback, even though there is no guarantee the addSpy had been called. My hope is that the variability is the cause of the flakiness.
So I switched it to wait for 2 addSpy callbacks (one for the help component, one for the chart), then verify the chart was added and then do any other further inspections of the chart. Should be less implementation-specific than examining arguments on the callbacks.