patch 8.2.4410: Vim9: some code not covered by tests

Problem:    Vim9: some code not covered by tests.
Solution:   Add a few more tests. Remove dead code.
diff --git a/src/testdir/test_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index e4bea07..8a51eac 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -1552,6 +1552,28 @@
   v9.CheckScriptFailure(lines, 'E1119', 4)
 
   lines =<< trim END
+      vim9script
+      var theList = [1, 2, 3]
+      def AddToList()
+        lockvar theList
+        theList += [4]
+      enddef
+      AddToList()
+  END
+  v9.CheckScriptFailure(lines, 'E741', 2)
+
+  lines =<< trim END
+      vim9script
+      var theList = [1, 2, 3]
+      def AddToList()
+        lockvar theList
+        add(theList, 4)
+      enddef
+      AddToList()
+  END
+  v9.CheckScriptFailure(lines, 'E741', 2)
+
+  lines =<< trim END
       var theList = [1, 2, 3]
       lockvar theList
   END