[wasm] Enable System.Xml.Linq.Tests.XTypeDescriptionProviderTests class

[MaximLipnin]

While enabling System.ComponentModel.TypeConverter.Tests on Browser WASM, I faced the failures in System.Xml.Linq.Tests.XTypeDescriptionProviderTests test class.

There are 7 test cases which mostly test the property-related logic for XElement and XAttribute classes (dotnet/corefx#33082).
Those classes are annotated with System.ComponentModel.TypeDescriptionProviderAttribute so that TypeDescriptor.GetProperties() can provide with some additional properties

runtime/src/libraries/System.ComponentModel.TypeConverter/src/MS/Internal/Xml/Linq/ComponentModel/XComponentModel.cs

Lines 36 to 61 in 6072e4d

	public override PropertyDescriptorCollection GetProperties(Attribute[] attributes)
	{
	PropertyDescriptorCollection properties = new PropertyDescriptorCollection(null);
	if (attributes == null)
	{
	if (typeof(T) == typeof(XElement))
	{
	properties.Add(new XElementAttributePropertyDescriptor());
	properties.Add(new XElementDescendantsPropertyDescriptor());
	properties.Add(new XElementElementPropertyDescriptor());
	properties.Add(new XElementElementsPropertyDescriptor());
	properties.Add(new XElementValuePropertyDescriptor());
	properties.Add(new XElementXmlPropertyDescriptor());
	}
	else if (typeof(T) == typeof(XAttribute))
	{
	properties.Add(new XAttributeValuePropertyDescriptor());
	}
	}
	foreach (PropertyDescriptor property in base.GetProperties(attributes))
	{
	properties.Add(property);
	}
	return properties;
	}
	}

.

The mentioned tests get a collection of property descriptors by calling something like below

var xel = new XElement("someElement");
var props = TypeDescriptor.GetProperties(xel);

Actually props contains only a collection of some general properties and doesn't include the additional ones.

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[ericstj]

cc @eerhardt looks like a linker correctness issue.