Test SerializationCodeGenerator.

XmlSerializerTests was mostly testing XmlSerializationReaderInterpreter and XmlSerializationWriterInterpreter.
Added XmlSerializerGeneratorTests, it forces the use of SerializationCodeGenerator while running XmlSerializerTests.

Fixed SerializationCodeGenerator issues that were producing invalid code.

Fixed serialization of XmlNode field with attribute XmlAnyElement.
**Fixes:**[#3211](https://bugzilla.xamarin.com/show_bug.cgi?id=3211)