patch 8.2.4835: Vim9: some lines not covered by tests

Problem:    Vim9: some lines not covered by tests.
Solution:   Add a few more tests.  Fix disassemble output.
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index 49ca29b..4cd78f6 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -1691,6 +1691,9 @@
   endfor
   assert_equal('yes no abc', getline(1))
 
+  setline(1, 'from')
+  v9.CheckDefExecFailure(['s/from/\=g:notexist/'], 'E121: Undefined variable: g:notexist')
+
   bwipe!
 
   v9.CheckDefFailure(['s/from/\="x")/'], 'E488:')