patch 9.1.1138: cmdline completion for :hi is too simplistic

Problem:  Existing cmdline completion for :highlight was barebone and
          only completed the highlight group names.

Solution: Implement full completion for the highlight group arguments
          such as guifg and cterm. If the user tries to complete
          immediately after the '=' (e.g. `hi Normal guifg=<Tab>`), the
          completion will fill in the existing value, similar to how
          cmdline completion for options work (Yee Cheng Chin).

closes: #16712

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index 748c2bd..49f8833 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -209,7 +209,7 @@
 
 func Test_echohl_completion()
   call feedkeys(":echohl no\<C-A>\<C-B>\"\<CR>", 'tx')
-  call assert_equal('"echohl NonText Normal none', @:)
+  call assert_equal('"echohl NONE NonText Normal', @:)
 endfunc
 
 func Test_syntax_arg_skipped()