[tests] Skip the TypeDescriptorCanary test in .NET.

[rolfbjarne]

It tests internal implementation details, and those details have now changed
(dotnet/runtime#39973). So just skip the entire test
for now.

Fixes this test failure:

DontLink.CommonDontLinkTest
    [FAIL] TypeDescriptorCanary : System.InvalidCastException : Specified cast is not valid. :    at DontLink.CommonDontLinkTest.TypeDescriptorCanary() in /Users/builder/jenkins/workspace/xamarin-macios-pr-builder/tests/linker/CommonDontLinkTest.cs:line 22
        at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

[monojenkins]

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