Update XML tests to register a code page provider

Required for the tests to pass on .NET Core.


Commit migrated from https://github.com/dotnet/corefx/commit/b48137e5b74d26310c97cae03e17c84cb4f25c7a

src/libraries/Common/tests/System.Xml.XPath/TestsWithEncoding/MiscWithEncodings.cs

@@ -17,8 +17,7 @@ public static partial class MiscWithEncodingTests
     {
         public static void TestInitialize()
         {
-            //This will work on Desktop although we need System.Text.Encoding.CodePages to make this work on CoreCLR
-            //Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+            Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
         }
 
         /// <summary>