patch 8.2.0670: cannot change window when evaluating 'completefunc'

Problem:    Cannot change window when evaluating 'completefunc'.
Solution:   Make a difference between not changing text or buffers and also
            not changing window.
diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index cec4acf..dbea901 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -491,7 +491,7 @@
   endfunc
   set completefunc=CompleteFunc2
   call setline(1, ['', 'abcd', ''])
-  call assert_fails('exe "normal 2G$a\<C-X>\<C-U>"', 'E565:')
+  call assert_fails('exe "normal 2G$a\<C-X>\<C-U>"', 'E578:')
 
   set completefunc&
   delfunc CompleteFunc