[lldb[test] TestCppUnionStaticMembers.py: XFAIL assertions on windows

These tests never worked since their introduction in
#68300

Author: Michael137

lldb/test/API/lang/cpp/union-static-data-members/TestCppUnionStaticMembers.py

@@ -27,6 +27,7 @@ def test_print_union(self):
                 name="val", value="137"
             )])
 
+    @expectedFailureWindows
     def test_expr_union_static_members(self):
         """Tests that frame variable and expr work
         for union static data members"""