patch 9.0.1798: The 'syntax' option has no completion.

Problem:  The 'syntax' option has no completion.
Solution: Add syntax option completion.

closes: #12900

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Doug Kearns <dougkearns@gmail.com>
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index fec8d2a..fc81cec 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -374,6 +374,12 @@
   call assert_equal('"set filetype=sshdconfig', @:)
   call feedkeys(":set filetype=a\<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal('"set filetype=' .. getcompletion('a*', 'filetype')->join(), @:)
+
+  " Expand values for 'syntax'
+  call feedkeys(":set syntax=sshdconfi\<Tab>\<C-B>\"\<CR>", 'xt')
+  call assert_equal('"set syntax=sshdconfig', @:)
+  call feedkeys(":set syntax=a\<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_equal('"set syntax=' .. getcompletion('a*', 'syntax')->join(), @:)
 endfunc
 
 func Test_set_option_errors()