Stub File.unlink without redefining it in the tempfile test

Redefining the method warns under RUBYOPT=-w, which CI uses, and the test expects nothing on stderr. Prepending a module leaves the original method in place.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_tempfile.rb

@@ -237,9 +237,11 @@ File.open(path, "w").close
 
   def test_finalizer_ignores_eacces_on_unlink
     assert_in_out_err("-r#{LIB_TEMPFILE_RB_PATH}", <<~'RUBY') do |(filename,*), (error,*)|
-      def File.unlink(*)
-        raise Errno::EACCES
-      end
+      File.singleton_class.prepend(Module.new do
+        def unlink(*)
+          raise Errno::EACCES
+        end
+      end)
       file = Tempfile.new("foo")
       puts file.path
     RUBY