Validate MessagePack enum values

[smoogipoo]

No description provided.

[smoogipoo]

The "WithVerify" part of this checks that the formatter is not null. Was just looking through the MsgPack source and this is kind of how they do it, plus I needed the non-nullability.

[bdach]

[Flags] enums need to be exempted from this because there not being "defined" is actually a feature if you have a union of 2 or more flags.

This is actually relevant because we have one of those that goes over the wire to spectator server and it's ReplayButtonState.

diff --git a/osu.Game.Tests/Online/TestMultiplayerMessagePackSerialization.cs b/osu.Game.Tests/Online/TestMultiplayerMessagePackSerialization.cs
index 6891b214d0..08e3e3a660 100644
--- a/osu.Game.Tests/Online/TestMultiplayerMessagePackSerialization.cs
+++ b/osu.Game.Tests/Online/TestMultiplayerMessagePackSerialization.cs
@@ -7,6 +7,7 @@
 using osu.Game.Online;
 using osu.Game.Online.Multiplayer;
 using osu.Game.Online.Multiplayer.MatchTypes.TeamVersus;
+using osu.Game.Replays.Legacy;
 
 namespace osu.Game.Tests.Online
 {
@@ -86,6 +87,9 @@ public void TestEnumValidation()
                 MessagePackSerializer.Deserialize<SimpleEnum?>(MessagePackSerializer.Serialize((SimpleEnum?)null, SignalRUnionWorkaroundResolver.OPTIONS), SignalRUnionWorkaroundResolver.OPTIONS));
             Assert.Throws<MessagePackSerializationException>(() =>
                 MessagePackSerializer.Deserialize<SimpleEnum?>(MessagePackSerializer.Serialize((SimpleEnum)100, SignalRUnionWorkaroundResolver.OPTIONS), SignalRUnionWorkaroundResolver.OPTIONS));
+
+            Assert.DoesNotThrow(() =>
+                MessagePackSerializer.Deserialize<ReplayButtonState>(MessagePackSerializer.Serialize(ReplayButtonState.Left1 | ReplayButtonState.Right2, SignalRUnionWorkaroundResolver.OPTIONS), SignalRUnionWorkaroundResolver.OPTIONS));
         }
 
         public enum SimpleEnum

The above fails when run.