patch 8.2.0049: command line completion not fully tested

Problem:    Command line completion not fully tested.
Solution:   Add more test cases.  Make help sorting stable. (Dominique Pelle,
            closes #5402)
diff --git a/src/testdir/test_help.vim b/src/testdir/test_help.vim
index 5dd937a..4f46a44 100644
--- a/src/testdir/test_help.vim
+++ b/src/testdir/test_help.vim
@@ -55,3 +55,8 @@
   call delete('Xruntime', 'rf')
   let &rtp = rtp_save
 endfunc
+
+func Test_help_completion()
+  call feedkeys(":help :undo\<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"help :undo :undoj :undol :undojoin :undolist', @:)
+endfunc