Fix formatting and unused using directives

Fixed missing blank line in `TestSceneMultiplayerMatchSubScreen.cs` and removed unused `using System;` in `BackgroundDataStoreProcessorTests.cs` to resolve code quality check failures. Verified with `dotnet build -warnaserror`.

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: google-labs-jules[bot]

osu.Game.Tests/Database/BackgroundDataStoreProcessorTests.cs

@@ -1,8 +1,6 @@
 // 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.
 
-#pragma warning disable IDE0005
-using System;
 using System.Linq;
 using NUnit.Framework;
 using osu.Framework.Allocation;

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiplayerMatchSubScreen.cs

@@ -69,6 +69,7 @@ private void load(GameHost host, AudioManager audio)
             });
         }
 
+
         public override void SetUpSteps()
         {
             base.SetUpSteps();