Fix LoadRequested sent to all users in multiplayer rooms

[smoogipoo]

This is part 2 to fixing ppy/osu#36045, and mentioned as bullet point 1 in ppy/osu#36121:

The multiplayer server sends LoadRequested to all users in the room regardless of their beatmap availability. This is an issue on its own that will be fixed separately.

I'm not 100% sure on this change in principle, and there are 3 paths that we can go down here:

1. This change.
2. Re-introduce the server-side "gameplay" group rather than using .Users(). This seems like it'd be a more complex change.
3. Continue sending LoadRequested to all users in the room and handle this locally in the client.

[smoogipoo]

The number of test changes here makes me feel a bit uneasy, as if option 3 from the OP is the better one to go down... Let's treat this PR as an RFC.

[bdach]

The number of test changes here makes me feel a bit uneasy

Without real full-stack testing I'm kind of unbothered by anything in here? Unsure whether this is because of lack of bothering capacity or there's really not much to see here.

[smoogipoo]

Maybe it's just me being a bit too critical.

[peppy]

@smoogipoo shall we give this another try? needs some conflict resolution.