patch 8.1.1810: popup_getoptions() is missing an entry for "mapping"

Problem:    Popup_getoptions() is missing an entry for "mapping".
Solution:   Add the entry.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index e5d1631..5a841e3 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -946,6 +946,7 @@
   call assert_equal(21, res.maxheight)
   call assert_equal(100, res.zindex)
   call assert_equal(1, res.fixed)
+  call assert_equal(1, res.mapping)
   if has('timers')
     call assert_equal(5000, res.time)
   endif