Refactor SessionsController test

This change is already merged, but I wanted to add some notes explaining some of the changes I made. Once my notes have been reviewed, this PR can be closed.
Also related to this line, I changed it to `toHaveBeenCalledWith` so we can do more validation on how `growlNotifications` is called. This makes the test more brittle because now a change to how long the failure message is shown can break the test, but for now that is okay with me. There are steps that can be taken to avoid this added brittleness, but they're more effort than is worth it for this PR.