The triangles intro tracks video time, which is not adjusted based on
the game's playback rate (ie. it runs in realtime even for headless
tests).
Maybe we want to make the triangles video adjust its rate along with
tests?
I guess the changes that involved `MultiplayerTestScene` having a test
user lookup cache caused this test case to false-pass silently.
Added an explicit assert which ensures the added user indeed has a null
`User` value.