patch 8.2.4407: Vim9: some code not covered by tests

Problem:    Vim9: some code not covered by tests.
Solution:   Add more tests.  Avoid giving two errors.  Remove dead code.
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index 10be691..e4bea07 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -1529,6 +1529,17 @@
 
   var lines =<< trim END
       vim9script
+      g:bl = 0z1122
+      lockvar g:bl
+      def Tryit()
+        g:bl[1] = 99
+      enddef
+      Tryit()
+  END
+  v9.CheckScriptFailure(lines, 'E741:', 1)
+
+  lines =<< trim END
+      vim9script
       var theList = [1, 2, 3]
       def SetList()
         theList[1] = 22