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_syntax.vim b/src/testdir/test_syntax.vim
index 21ee8dc..389294e 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -180,6 +180,11 @@
   call assert_match('^"syn match Boolean Character ', @:)
 endfunc
 
+func Test_echohl_completion()
+  call feedkeys(":echohl no\<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"echohl NonText Normal none', @:)
+endfunc
+
 func Test_syntax_arg_skipped()
   syn clear
   syntax case ignore