patch 8.2.2427: can still switch windows for 'completefunc'

Problem:    Can still switch windows for 'completefunc'.
Solution:   Also disallow switching windows for other completions.
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index 2d0625b..c8f8820 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -367,11 +367,9 @@
   setlocal completefunc=DummyCompleteTwo
   call setline(1, 'two')
   /^two
-  call assert_fails('call feedkeys("A\<C-X>\<C-U>\<C-N>\<Esc>", "x")', 'E839:')
-  call assert_notequal(winid, win_getid())
-  q!
+  call assert_fails('call feedkeys("A\<C-X>\<C-U>\<C-N>\<Esc>", "x")', 'E565:')
   call assert_equal(winid, win_getid())
-  call assert_equal('two', getline(1))
+  call assert_equal('twodef', getline(1))
   q!
 endfunc