patch 8.0.0157: no completion for :syntax spell and :syntax sync

Problem:    No command line completion for ":syntax spell" and ":syntax sync".
Solution:   Implement the completion. (Dominique Pelle)
diff --git a/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index 7cb67e6..23ef154 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -150,6 +150,12 @@
   call feedkeys(":syn case \<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_equal('"syn case ignore match', @:)
 
+  call feedkeys(":syn spell \<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"syn spell default notoplevel toplevel', @:)
+
+  call feedkeys(":syn sync \<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"syn sync ccomment clear fromstart linebreaks= linecont lines= match maxlines= minlines= region', @:)
+
   call feedkeys(":syn list \<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_match('^"syn list Boolean Character ', @:)