patch 9.0.1211: storing value in interface member does not always work

Problem:    Storing value in interface member does not always work.
Solution:   Convert the index on the interface to the index on the object.
diff --git a/src/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index ee10a3c..dadd3e4 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -876,14 +876,14 @@
       vim9script
 
       interface Result
-        this.label: string
+        public this.label: string
         this.errpos: number
       endinterface
 
       # order of members is opposite of interface
       class Failure implements Result
         this.errpos: number = 42
-        this.label: string = 'label'
+        public this.label: string = 'label'
       endclass
 
       def Test()
@@ -891,6 +891,10 @@
 
         assert_equal('label', result.label)
         assert_equal(42, result.errpos)
+
+        result.label = 'different'
+        assert_equal('different', result.label)
+        assert_equal(42, result.errpos)
       enddef
 
       Test()