patch 9.0.1665: empty CmdlineEnter autocommand causes errors in Ex mode

Problem:    Empty CmdlineEnter autocommand causes errors in Ex mode.
Solution:   Save and restore ex_pressedreturn. (Christian Brabandt,
            closes # 12581, closes #12578)
diff --git a/src/testdir/test_ex_mode.vim b/src/testdir/test_ex_mode.vim
index d03ec8f..59c28f3 100644
--- a/src/testdir/test_ex_mode.vim
+++ b/src/testdir/test_ex_mode.vim
@@ -239,6 +239,12 @@
 
   au! CmdLineEnter
   delfunc ExEnterFunc
+
+  au CmdlineEnter * :
+  call feedkeys("gQecho 1\r", 'xt')
+
+  au! CmdlineEnter
+
   quit
 endfunc