patch 8.0.0237: when 'wildoptions' is "tagfile" completion may not work

Problem:    When setting wildoptions=tagfile the completion context is not set
            correctly. (desjardins)
Solution:   Check for EXPAND_TAGS_LISTFILES. (Christian Brabandt, closes #1399)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 279a168..fcfce73 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -295,3 +295,14 @@
   2;')
   quit
 endfunc
+
+func Test_cmdline_complete_wildoptions()
+  help
+  call feedkeys(":tag /\<c-a>\<c-b>\"\<cr>", 'tx')
+  let a = join(sort(split(@:)),' ')
+  set wildoptions=tagfile
+  call feedkeys(":tag /\<c-a>\<c-b>\"\<cr>", 'tx')
+  let b = join(sort(split(@:)),' ')
+  call assert_equal(a, b)
+  bw!
+endfunc