[tests] Ignore the XmlSerializationTest.Bug1820_GenericList test in .NET due to a linker bug.

Ref: https://github.com/mono/linker/issues/1454

Fixes this link sdk test failure:

    LinkSdk.Serialization.XmlSerializationTest
        [FAIL] Bug1820_GenericList : System.ArgumentNullException : Value cannot be null. (Parameter 'obj')
            at System.Reflection.Emit.ModuleBuilder.GetToken(MemberInfo member, Boolean create_open_instance)
            at System.Reflection.Emit.ModuleBuilderTokenGenerator.GetToken(MemberInfo member, Boolean create_open_instance)
            at System.Reflection.Emit.ILGenerator.Emit(OpCode opcode, ConstructorInfo con)
            at System.Xml.Serialization.CodeGenerator.Call(ConstructorInfo ctor)
            at System.Xml.Serialization.XmlSerializationILGen.GenerateSerializerContract(String className, XmlMapping[] xmlMappings, Type[] types, String readerType, String[] readMethods, String writerType, String[] writerMethods, Dictionary`2 serializers)
            at System.Xml.Serialization.TempAssembly.GenerateRefEmitAssembly(XmlMapping[] xmlMappings, Type[] types, String defaultNamespace)
            at System.Xml.Serialization.TempAssembly..ctor(XmlMapping[] xmlMappings, Type[] types, String defaultNamespace, String location)
            at System.Xml.Serialization.XmlSerializer.GenerateTempAssembly(XmlMapping xmlMapping, Type type, String defaultNamespace, String location)
            at System.Xml.Serialization.XmlSerializer.GenerateTempAssembly(XmlMapping xmlMapping, Type type, String defaultNamespace)
            at System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace)
            at System.Xml.Serialization.XmlSerializer..ctor(Type type)
            at LinkSdk.Serialization.XmlSerializationTest.Response.get_Serializer() in [...]/xamarin-macios/tests/linker/ios/link sdk/Bug1820Test.cs:line 38
            at LinkSdk.Serialization.XmlSerializationTest.Response.Deserialize(String xml) in [...]/xamarin-macios/tests/linker/ios/link sdk/Bug1820Test.cs:line 48
            at LinkSdk.Serialization.XmlSerializationTest.Bug1820_GenericList() in [...]/xamarin-macios/tests/linker/ios/link sdk/Bug1820Test.cs:line 166
            at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

tests/linker/ios/link sdk/Bug1820Test.cs

@@ -148,6 +148,9 @@ namespace LinkSdk.Serialization {
 			}
 		}
 		
+#if NET
+		[Ignore ("https://github.com/mono/linker/issues/1454")]
+#endif
 		[Test]
 		// http://bugzilla.xamarin.com/show_bug.cgi?id=1820
 		// note: this also test the linker (5.1+) ability not to remove 'unused' XML setters and .ctors used for serialization