Add missing license header

***NO_CI***

Author: josesimoes

Tests/NFUnitTestRegex/ComplexExpressionsTests.cs

@@ -1,5 +1,10 @@
-using nanoFramework.TestFramework;
-using System;
+//
+// Copyright (c) .NET Foundation and Contributors
+// Portions Copyright (c) Microsoft Corporation.  All rights reserved.
+// See LICENSE file in the project root for full license information.
+//
+
+using nanoFramework.TestFramework;
 using System.Diagnostics;
 using System.Text.RegularExpressions;