Fix admin test arena (#35444)

* Fix admin test arena

* Add to GridsLoadableTest

* QueueDel map, remove nullable

---------

Co-authored-by: ElectroJr <leonsfriedrich@gmail.com>

Content.IntegrationTests/Tests/PostMapInitTest.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using Content.Server.Administration.Systems;
 using Content.Server.GameTicking;
 using Content.Server.Maps;
 using Content.Server.Shuttles.Components;
@@ -38,7 +39,8 @@ namespace Content.IntegrationTests.Tests
 
         private static readonly string[] Grids =
         {
-            "/Maps/centcomm.yml"
+            "/Maps/centcomm.yml",
+            AdminTestArenaSystem.ArenaMapPath
         };
 
         private static readonly string[] DoNotMapWhitelist =