[tests] Ignore CommonLinkSdkTest.TypeDescriptor_A7793 in .NET due to a linker bug.

Ref: https://github.com/mono/linker/issues/1451

Fixes this link sdk test failure:

    LinkSdk.CommonLinkSdkTest
        [FAIL] TypeDescriptor_A7793 : System.MissingMethodException : Default constructor not found for type System.ComponentModel.DateTimeOffsetConverter
            at System.RuntimeType.CreateInstanceMono(Boolean nonPublic, Boolean wrapExceptions)
            at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean wrapExceptions, Boolean skipCheckThis, Boolean fillCache)
            at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, Boolean wrapExceptions)
            at System.Activator.CreateInstance(Type type, Boolean nonPublic, Boolean wrapExceptions)
            at System.Activator.CreateInstance(Type type, Boolean nonPublic)
            at System.Activator.CreateInstance(Type type)
            at System.ComponentModel.ReflectTypeDescriptionProvider.CreateInstance(Type objectType, Type callingType)
            at System.ComponentModel.ReflectTypeDescriptionProvider.SearchIntrinsicTable(Hashtable table, Type callingType)
            at System.ComponentModel.ReflectTypeDescriptionProvider.ReflectedTypeData.GetConverter(Object instance)
            at System.ComponentModel.ReflectTypeDescriptionProvider.GetConverter(Type type, Object instance)
            at System.ComponentModel.TypeDescriptor.TypeDescriptionNode.DefaultTypeDescriptor.System.ComponentModel.ICustomTypeDescriptor.GetConverter()
            at System.ComponentModel.TypeDescriptor.GetConverter(Type type)
            at LinkSdk.CommonLinkSdkTest.TypeDescriptor_A7793() in /Users/rolf/work/maccore/whatever/xamarin-macios/tests/linker/CommonLinkSdkTest.cs:line 21
            at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

tests/linker/CommonLinkSdkTest.cs

@@ -13,6 +13,9 @@ namespace LinkSdk {
 	[TestFixture]
 	// we want the test to be availble if we use the linker
 	[Preserve (AllMembers = true)]
+#if NET
+	[Ignore ("Type converters are linked away: https://github.com/mono/linker/issues/1451")]
+#endif
 	public class CommonLinkSdkTest {
 
 		[Test]