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_script.vim b/src/testdir/test_vim9_script.vim
index 9993c5e..3c3434d 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -52,6 +52,15 @@
 
   bwipe!
 
+  lines =<< trim END
+      set cpo+=-
+      :1,999
+  END
+  v9.CheckDefExecAndScriptFailure(lines, 'E16:', 2)
+  set cpo&vim
+
+  v9.CheckDefExecAndScriptFailure([":'x"], 'E20:', 1)
+
   # won't generate anything
   if false
     :123
@@ -1726,6 +1735,9 @@
   v9.CheckDefFailure(['execute xxx'], 'E1001:', 1)
   v9.CheckDefExecFailure(['execute "tabnext " .. 8'], 'E475:', 1)
   v9.CheckDefFailure(['execute "cmd"# comment'], 'E488:', 1)
+  if has('channel')
+    v9.CheckDefExecFailure(['execute test_null_channel()'], 'E908:', 1)
+  endif
 enddef
 
 def Test_execute_cmd_vimscript()