patch 9.0.0283: cannot complete "syn list @cluster"

Problem:    Cannot complete "syn list @cluster".
Solution:   Recognize and handle "list @". (Björn Linse, closes #10990)
diff --git a/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index 93e56d7..7f1e5f0 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -201,6 +201,10 @@
 
   call feedkeys(":syn match \<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_match('^"syn match Boolean Character ', @:)
+
+  syn cluster Aax contains=Aap
+  call feedkeys(":syn list @A\<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_match('^"syn list @Aax', @:)
 endfunc
 
 func Test_echohl_completion()