patch 8.2.0283: Vim9: failing to load script var not tested

Problem:    Vim9: failing to load script var not tested.
Solution:   Add more tests.   Fix using s: in old script.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 235aee2..1b04d02 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -726,6 +726,9 @@
 
   call CheckDefFailure("let x = &notexist", 'E113:')
   call CheckDefExecFailure("&grepprg = [343]", 'E1051:')
+
+  call CheckDefExecFailure("echo s:doesnt_exist", 'E121:')
+  call CheckDefExecFailure("echo g:doesnt_exist", 'E121:')
 endfunc
 
 let g:Funcrefs = [function('add')]