patch 8.2.0810: error when appending "tagfile" to 'wildoptions'

Problem:    Error when appending "tagfile" to 'wildoptions'.
Solution:   use flags P_ONECOMMA and P_NODUP. (Dmitri Vereshchagin,
            closes #6105)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index ce925d8..17ae094 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -60,6 +60,13 @@
   set wildchar&
 endfunc
 
+func Test_wildoptions()
+  set wildoptions=
+  set wildoptions+=tagfile
+  set wildoptions+=tagfile
+  call assert_equal('tagfile', &wildoptions)
+endfunc
+
 func Test_options_command()
   let caught = 'ok'
   try