Added copyright header

Author: darrelmiller

test/Microsoft.OpenApi.Readers.Tests/TestCustomExtension.cs

@@ -1,14 +1,10 @@
-using FluentAssertions;
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. 
+
+using FluentAssertions;
 using Microsoft.OpenApi.Any;
 using Microsoft.OpenApi.Interfaces;
-using Microsoft.OpenApi.Validations;
-using Microsoft.OpenApi.Validations.Rules;
 using Microsoft.OpenApi.Writers;
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using Xunit;
 
 namespace Microsoft.OpenApi.Readers.Tests