Don't make the test a MonoBehavior (#3505)

com.unity.ml-agents/Tests/Editor/DemonstrationTests.cs

 namespace MLAgents.Tests
 {
     [TestFixture]
-    public class DemonstrationTests : MonoBehaviour
+    public class DemonstrationTests
     {
         const string k_DemoDirectory = "Assets/Demonstrations/";
         const string k_ExtensionType = ".demo";