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_ins_complete.vim b/src/testdir/test_ins_complete.vim
index 489c1d9..613e952 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -548,7 +548,7 @@
   endfunc
   set completefunc=CompleteFunc
   call setline(1, ['', 'abcd', ''])
-  call assert_fails('exe "normal 2G$a\<C-X>\<C-U>"', 'E840:')
+  call assert_fails('exe "normal 2G$a\<C-X>\<C-U>"', 'E578:')
 
   " delete text when called for the second time
   func CompleteFunc2(findstart, base)
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