vim-patch:8.1.0258: not enough testing for the CompleteDone event

Problem:    Not enough testing for the CompleteDone event.
Solution:   Add a test. (closes vim/vim#3297)
af559d2c9f

src/nvim/testdir/test_ins_complete.vim

@@ -142,6 +142,19 @@ function Test_CompleteDoneDict()
   au! CompleteDone
 endfunc
 
+func Test_CompleteDone_undo()
+  au CompleteDone * call append(0, "prepend1")
+  new
+  call setline(1, ["line1", "line2"])
+  call feedkeys("Go\<C-X>\<C-N>\<CR>\<ESC>", "tx")
+  call assert_equal(["prepend1", "line1", "line2", "line1", ""],
+              \     getline(1, '$'))
+  undo
+  call assert_equal(["line1", "line2"], getline(1, '$'))
+  bwipe!
+  au! CompleteDone
+endfunc
+
 function! s:CompleteDone_CompleteFuncDictNoUserData( findstart, base )
   if a:findstart
     return 0