Reverted changes to AddNetworkPrefabTests and fixed instabilities caused by NetworkManager.Singleton (can we get rid of that completely soon?)

Author: ShadauxCat

com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs

@@ -1292,11 +1292,6 @@ private void OnDestroy()
             ShutdownInternal();
 
             UnityEngine.SceneManagement.SceneManager.sceneUnloaded -= OnSceneUnloaded;
-
-            if (Singleton == this)
-            {
-                Singleton = null;
-            }
         }
 
         private void DisconnectRemoteClient(ulong clientId)
@@ -1407,6 +1402,11 @@ internal void ShutdownInternal()
 
             this.UnregisterAllNetworkUpdates();
 
+            if (Singleton == this)
+            {
+                Singleton = null;
+            }
+
             if (NetworkTickSystem != null)
             {
                 NetworkTickSystem.Tick -= OnNetworkManagerTick;

com.unity.netcode.gameobjects/TestHelpers/Runtime/NetcodeIntegrationTestHelpers.cs

@@ -309,6 +309,7 @@ public static void Destroy()
             foreach (var networkManager in NetworkManagerInstances)
             {
                 networkManager.Shutdown();
+                networkManager.ShutdownInternal();
                 s_Hooks.Remove(networkManager);
             }
 

com.unity.netcode.gameobjects/Tests/Runtime/AddNetworkPrefabTests.cs

@@ -11,7 +11,6 @@ public class AddNetworkPrefabTest : NetcodeIntegrationTest
     {
         public class EmptyComponent : NetworkBehaviour
         {
-
         }
         protected override int NumberOfClients => 1;
 
@@ -22,18 +21,18 @@ protected override IEnumerator OnSetup()
             // Host is irrelevant, messages don't get sent to the host "client"
             m_UseHost = false;
 
-            yield return null;
-        }
-
-        protected override void OnServerAndClientsCreated()
-        {
             m_Prefab = new GameObject("Object");
             var networkObject = m_Prefab.AddComponent<NetworkObject>();
             m_Prefab.AddComponent<EmptyComponent>();
 
             // Make it a prefab
             NetcodeIntegrationTestHelpers.MakeNetworkObjectTestPrefab(networkObject);
 
+            yield return null;
+        }
+
+        protected override void OnServerAndClientsCreated()
+        {
             m_ServerNetworkManager.NetworkConfig.SpawnTimeout = 0;
             m_ServerNetworkManager.NetworkConfig.ForceSamePrefabs = false;
             foreach (var client in m_ClientNetworkManagers)