Add cloning to MultiplayerPlaylistItem, and tests using fake data

[smoogipoo]

This is intended to fix a separate issue I've found server-side, where MultiplayerPlaylistItem's are cloned manually: https://github.com/ppy/osu-server-spectator/blob/8c3c87039659d623870f77bf7bf3d1610f202f85/osu.Server.Spectator/Hubs/Multiplayer/MultiplayerQueue.cs#L254-L263
Given that this has come up a few times now, I think it's best to add cloning and supporting tests.

For testing, I've added the Bogus library. It seems well supported and MIT. Its strict mode tells us when a property isn't being touched by the test, which should catch any missed updates to these methods.

The checksum change is irrelevant to the above fix - it was noticed separately as a consequence of the tests.

[bdach]

Probably gonna need @peppy's ok for the inclusion on this. It seems mostly legit on a quick look although there's some DRM-shaped stuff happening because the library has paid extensions included, and also some other somewhat dodgy stuff.

[smoogipoo]

One alternative that just came to my mind, is I can replace it with

public MultiplayerPlaylistItem Clone()
{
    MultiplayerPlaylistItem clone = (MultiplayerPlaylistItem)MemberwiseClone();
    clone.RequiredMods = RequiredMods.ToArray();
    clone.AllowedMods = AllowedMods.ToArray();
    return clone;
}

And we can live without tests under the premise that MemberwiseClone() has been properly tested by .NET itself. Thoughts?

[smoogipoo]

Ah but the tests also test MPI -> PI -> MPI... Think I'll replace with MemberwiseClone() in any case. :/

I hope these classes can be interfaced or merged, at some point...

[peppy]

I think it's okay since it's limited to the tests project.

[bdach]

no major objections I guess, requesting @peppy review for the new library inclusion