patch 9.0.0275: BufEnter not triggered when using ":edit" in "nofile" buffer

Problem:    BufEnter not triggered when using ":edit" in "nofile" buffer.
Solution:   Let readfile() return NOTDONE. (closes #10986)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index c043a35..4b09285 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -681,9 +681,19 @@
   " On MS-Windows we can't edit the directory, make sure we wipe the right
   " buffer.
   bwipe! Xdir
-
   call delete('Xdir', 'd')
   au! BufEnter
+
+  " Editing a "nofile" buffer doesn't read the file but does trigger BufEnter
+  " for historic reasons.
+  new somefile
+  set buftype=nofile
+  au BufEnter somefile call setline(1, 'some text')
+  edit
+  call assert_equal('some text', getline(1))
+
+  bwipe!
+  au! BufEnter
 endfunc
 
 " Closing a window might cause an endless loop