vim-patch:8.2.4200: some tests do not clean up properly

Problem:    Some tests do not clean up properly.
Solution:   Delete created files. (Yegappan Lakshmanan, closes vim/vim#9611)
7e765a39b7

Omit test_filetype.vim: already ported
Omit test_vim9_import.vim: N/A

test/functional/legacy/messages_spec.lua

@@ -65,5 +65,6 @@ describe('messages', function()
       {1:~                                       }|
       'b' written                             |
     ]])
+    os.remove('b.txt')
   end)
 end)