vim-patch:9.0.0092: plugins cannot change v:completed_item (#19542)

Problem:    Plugins cannot change v:completed_item.
Solution:   Make v:completed_item writeable. (Shougo Matsushita,
            closes vim/vim#10801)
61021aa318

src/nvim/testdir/test_ins_complete.vim

@@ -336,6 +336,19 @@ func Test_CompleteDone_undo()
   au! CompleteDone
 endfunc
 
+func Test_CompleteDone_modify()
+  let value = {
+        \ 'word': '',
+        \ 'abbr': '',
+        \ 'menu': '',
+        \ 'info': '',
+        \ 'kind': '',
+        \ 'user_data': '',
+        \ }
+  let v:completed_item = value
+  call assert_equal(v:completed_item, value)
+endfunc
+
 func CompleteTest(findstart, query)
   if a:findstart
     return col('.')