Refactor song select test methods to be resilient to slow realm callbacks

[peppy]

As brought to light by https://gist.github.com/smoogipoo/56eda7ab56b9d1966556f2ca7a80a847.

[bdach]

I've ran this locally with the diff from the gist in OP and got this sort of error back once or twice:

TearDown : System.AggregateException : One or more errors occurred. (Specified argument was out of the range of valid values. (Parameter 'Get from RealmResults index:5 beyond range of:0'))
  ----> System.ArgumentOutOfRangeException : Specified argument was out of the range of valid values. (Parameter 'Get from RealmResults index:5 beyond range of:0')
--TearDown
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait()
   at osu.Framework.Extensions.TaskExtensions.WaitSafely(Task task)
   at osu.Framework.Testing.TestScene.checkForErrors()
   at osu.Framework.Testing.TestScene.RunTestsFromNUnit()
--ArgumentOutOfRangeException
   at Realms.ResultsHandle.GetValueAtIndex(Int32 index, Realm realm)
   at Realms.RealmResults`1.GetValueAtIndex(Int32 index)
   at osu.Game.Screens.Select.BeatmapCarousel.beatmapSetsChanged(IRealmCollection`1 sender, ChangeSet changes, Exception error) in /home/dachb/Dokumenty/opensource/osu/osu.Game/Screens/Select/BeatmapCarousel.cs:line 261
   at osu.Game.Database.RealmObjectExtensions.<>c__DisplayClass11_1`1.<QueryAsyncWithNotifications>b__2(Object _) in /home/dachb/Dokumenty/opensource/osu/osu.Game/Database/RealmObjectExtensions.cs:line 291

Not sure whether it's a separate issue yet.

[bdach]

So the crash I'm seeing boils down to the notification subscription itself, it seems:

Looks like the ChangeSet and the RealmResults are out of sync, presumably because of the sleep added in the gist to repro the other failures - I don't know how to reasonably explain it otherwise, other than "upstream bug". That would make this crash out of scope here I think, but also means that there's possibly a fundamental unsafety in posting the callback as there is apparently no guarantee that the ChangeSet is still going to be valid once the posted callback is actually executed... Which is worrying but still out of scope.