corrected using statements for standards check

Author: michalChrobot

com.unity.netcode.gameobjects/Tests/Runtime/DocumentationCodeSamples/NetworkVariable/CustomSerializationDocsTests.cs

@@ -1,6 +1,8 @@
 using System.Collections;
 using System.Text;
+#if ENABLE_CORECLR
 using NUnit.Framework;
+#endif
 using Unity.Netcode;
 using Unity.Netcode.TestHelpers.Runtime;
 using UnityEngine;

com.unity.netcode.gameobjects/Tests/Runtime/NetworkVariable/NetworkVariableAnticipationTests.cs

@@ -3,7 +3,6 @@
 using NUnit.Framework;
 using Unity.Netcode.TestHelpers.Runtime;
 using UnityEngine;
-using UnityEngine.TestTools;
 
 namespace Unity.Netcode.RuntimeTests
 {

com.unity.netcode.gameobjects/Tests/Runtime/NetworkVariable/OwnerPermissionTests.cs

@@ -1,6 +1,8 @@
 using System.Collections;
 using System.Collections.Generic;
+#if ENABLE_CORECLR
 using NUnit.Framework;
+#endif
 using Unity.Netcode.TestHelpers.Runtime;
 using UnityEngine;
 using UnityEngine.TestTools;