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.
The previous version tried to keep both normal multiplayer and team
multiplayer results as one screen, but didn't check that team-specific
components aren't null in `LoadComplete()`.
To decrease number of conditional, split off the team results screen to
a separate implementation, and choose one or the other at push time in
`MultiplayerPlayer`, depending on team count.