Skip variadic template test on Windows

Author: hlopko

test/Interop/Cxx/templates/class-template-variadic.swift

@@ -1,6 +1,9 @@
 // RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-cxx-interop)
 //
 // REQUIRES: executable_test
+//
+// We can't yet call member functions correctly on Windows (SR-13129).
+// XFAIL: OS=windows-msvc
 
 import ClassTemplateVariadic
 import StdlibUnittest