test: NetworkTickSystem test (#630)

* test: NetworkTickSystem test

* test: NetworkTickSystem test, removed unneeded code

Author: jeffreyrainy

com.unity.multiplayer.mlapi/Tests/Runtime/TickSystemTests.cs

@@ -0,0 +1,51 @@
+using System;
+using System.Collections;
+using UnityEngine;
+using UnityEngine.TestTools;
+using NUnit.Framework;
+
+namespace MLAPI.RuntimeTests
+{
+    public class TickSystemTests: IDisposable
+    {
+        private NetworkTickSystem m_TickSystem = null;
+        private float m_TestDuration = 5.0f;
+        private float m_SleepInterval = 0.001f;
+        private float m_TickInterval = 0.010f;
+
+        public void Dispose()
+        {
+            m_TickSystem.Dispose();
+            m_TickSystem = null;
+            NetworkUpdateLoop.UnregisterLoopSystems();
+        }
+
+        [UnityTest]
+        public IEnumerator VerifyTickSystem()
+        {
+            m_TickSystem = new NetworkTickSystem(m_TickInterval);
+
+            ushort tick0 = m_TickSystem.GetTick();
+            ushort lastTick = tick0;
+            float t0 = Time.unscaledTime;
+            float t1;
+
+            do
+            {
+                t1 = Time.unscaledTime;
+                ushort tick = m_TickSystem.GetTick();
+
+                Assert.IsTrue(tick >= lastTick); // check monotonicity of ticks
+
+                lastTick = tick;
+                yield return new WaitForSeconds(m_SleepInterval);
+            } while (t1 - t0 <= m_TestDuration);
+
+            int ticks = lastTick - tick0;
+            int expectedTicks = (int)(m_TestDuration / m_TickInterval);
+
+            // check overall number of ticks is within one tick of the expected value
+            Assert.IsTrue(Math.Abs(expectedTicks - ticks) < 2);
+        }
+    }
+}