Fix ILLink descriptor: update assembly name from xunit.assert to xunit.v3.assert

The ILLinker descriptor referenced the old assembly name, causing
NativeAOT and trimming builds to fail with IL2007/MSB3073 errors.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: agocke

eng/testing/ILLink.Descriptor.xunit.xml

@@ -15,7 +15,7 @@
     <namespace fullname="Xunit" />
     <namespace fullname="Xunit.Sdk" />
   </assembly>
-  <assembly fullname="xunit.assert">
+  <assembly fullname="xunit.v3.assert">
     <type fullname="Xunit.Sdk.CollectionTracker">
       <method name="CompareTypedSets" />
     </type>