patch 8.2.4484: Vim9: some error messages are not tested

Problem:    Vim9: some error messages are not tested.
Solution:   Add a few more test cases.  Delete dead code.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index f9cb62c..4aad9c9 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -2782,6 +2782,23 @@
 
   v9.CheckDefAndScriptSuccess(lines)
 
+  lines =<< trim END
+      vim9script
+
+      def PosIdx(s: string): string
+        return s[1]
+      enddef
+      def NegIdx(s: string): string
+        return s[-1]
+      enddef
+
+      set enc=latin1
+      assert_equal("\xe4", PosIdx("a\xe4\xe5"))
+      assert_equal("\xe5", NegIdx("a\xe4\xe5"))
+      set enc=utf-8
+  END
+  v9.CheckScriptSuccess(lines)
+
   v9.CheckDefExecAndScriptFailure(['echo g:testblob[2]'], 'E979:', 1)
   v9.CheckDefExecAndScriptFailure(['echo g:testblob[-3]'], 'E979:', 1)