Move UserLookupCache inside dependencies

osu.Game/Tests/Visual/TestUserLookupCache.cs

@@ -0,0 +1,19 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Threading;
+using System.Threading.Tasks;
+using osu.Game.Database;
+using osu.Game.Users;
+
+namespace osu.Game.Tests.Visual
+{
+    public class TestUserLookupCache : UserLookupCache
+    {
+        protected override Task<User> ComputeValueAsync(int lookup, CancellationToken token = default) => Task.FromResult(new User
+        {
+            Id = lookup,
+            Username = $"User {lookup}"
+        });
+    }
+}