At some point during the leaderboard data refactor, this test scene
became completely unusable. This change rewrites it to use
`SoloGameplayLeaderboardProvider` to be able to witness the user score
being resorted and advancing through the leaderboard.
I'd have preferred a `get; init;` property but tests were also attached
at the hip to the public bindable. Without some extra composition this
is the best that I can do.