patch 8.1.0559: command line completion not sufficiently tested

Problem:    Command line completion not sufficiently tested.
Solution:   Add more tests. (Dominique Pelle, closes #3622)
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index a85799e..de14b81 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -59,3 +59,8 @@
 
   set cmdheight&
 endfunction
+
+func Test_message_completion()
+  call feedkeys(":message \<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"message clear', @:)
+endfunc