patch 9.0.1483: += operator does not work on class member

Problem:    += operator does not work on class member.
Solution:   Do not skip as if "this." was used. (Christian Brabandt,
            closes #12263)
diff --git a/src/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index a8f161d..1aee9e0 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -401,6 +401,13 @@
       var f =  Foo.new(3)
       f.Add(17)
       assert_equal(20, f.x)
+
+      def AddToFoo(obj: Foo)
+        obj.x += 3
+      enddef
+
+      AddToFoo(f)
+      assert_equal(23, f.x)
   END
   v9.CheckScriptSuccess(lines)
 enddef