patch 8.2.0285: unused error message; cannot create s:var

Problem:    Unused error message. Cannot create s:var.
Solution:   Remove the error message. Make assignment to s:var work.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 1fb08ab..ac721a8 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -65,6 +65,8 @@
   assert_equal('xxxyyy', s:appendToMe)
   s:addToMe += 222
   assert_equal(333, s:addToMe)
+  s:newVar = 'new'
+  assert_equal('new', s:newVar)
 enddef
 
 func Test_assignment_failure()