fix the multi tfm test to not build net20

TestAssets/TestProjects/TestLibraryWithTwoFrameworks/Program.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Xml;
+
+namespace ConsoleApplication
+{
+    public class Program
+    {
+        public static void Main()
+        {
+            Console.WriteLine("Hello World!");
+#if NET20 || NET35 || NET45 || NET461
+            // Force XmlDocument to be used
+            var doc = new XmlDocument();
+#endif
+        }
+    }
+}