Added using directive

Author: michalChrobot

com.unity.netcode.gameobjects/Tests/Editor/Timing/NetworkTimeTests.cs

@@ -3,6 +3,7 @@
 using System.Linq;
 using NUnit.Framework;
 using UnityEngine;
+using UnityEngine.TestTools;
 using Random = System.Random;
 
 namespace Unity.Netcode.EditorTests