[tests] Ignore linkall's XmlSerializationTest because .NET's linker doesn't support Xml serialization yet.
Ref: https://github.com/dotnet/runtime/issues/41389#issuecomment-682668670
Fixes this test failure:
LinkAll.Serialization.Xml.XmlSerializationTest
[FAIL] GenericProperty_Bug5543 : System.InvalidOperationException : There was an error reflecting type 'LinkAll.Serialization.Xml.XmlResult`1[System.String]'.
----> System.InvalidOperationException : There was an error reflecting property 'Result'.
----> System.InvalidOperationException : You need to add XmlChoiceIdentifierAttribute to the 'Result' member.
at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(TypeModel model, String ns, ImportContext context, String dataType, XmlAttributes a, Boolean repeats, Boolean openModel, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(TypeModel model, String ns, ImportContext context, String dataType, XmlAttributes a, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportElement(TypeModel model, XmlRootAttribute root, String defaultNamespace, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(Type type, XmlRootAttribute root, String defaultNamespace)
at System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace)
at System.Xml.Serialization.XmlSerializer..ctor(Type type)
at LinkAll.Serialization.Xml.XmlSerializationTest.GenericProperty_Bug5543() in /Users/rolf/work/maccore/whatever/xamarin-macios/tests/linker/ios/link all/XmlSerializationTest.cs:line 52
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
--InvalidOperationException
at System.Xml.Serialization.XmlReflectionImporter.InitializeStructMembers(StructMapping mapping, StructModel model, Boolean openModel, String typeName, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportStructLikeMapping(StructModel model, String ns, Boolean openModel, XmlAttributes a, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(TypeModel model, String ns, ImportContext context, String dataType, XmlAttributes a, Boolean repeats, Boolean openModel, RecursionLimiter limiter)
--InvalidOperationException
at System.Xml.Serialization.XmlReflectionImporter.CheckAmbiguousChoice(XmlAttributes a, Type accessorType, String accessorName)
at System.Xml.Serialization.XmlReflectionImporter.ImportAccessorMapping(MemberMapping accessor, FieldModel model, XmlAttributes a, String ns, Type choiceIdentifierType, Boolean rpc, Boolean openModel, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportFieldMapping(StructModel parent, FieldModel model, XmlAttributes a, String ns, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.InitializeStructMembers(StructMapping mapping, StructModel model, Boolean openModel, String typeName, RecursionLimiter limiter)
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