patch 8.2.3859: Vim9: some code lines not tested

Problem:    Vim9: some code lines not tested.
Solution:   Add a few specific tests.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index af80935..fe985de 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -2963,7 +2963,7 @@
 
 def Test_for_loop_script_var()
   # cannot use s:var in a :def function
-  CheckDefFailure(['for s:var in range(3)', 'echo 3'], 'E461:')
+  CheckDefFailure(['for s:var in range(3)', 'echo 3'], 'E1254:')
 
   # can use s:var in Vim9 script, with or without s:
   var lines =<< trim END