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.
This is a note for myself, or anyone who should happen to look into it. I don't think the next expectation should be needed for this test if things are configured/isolated correctly. The SessionController has nothing to do with the navbar_page_header_layout, so if things were properly isolated we could test the SessionController without having to worry about other unrelated things. That said, there's probably a fair amount of effort required to actually isolate things properly for testing, so I'm punting on it for now.