patch 8.1.1537: using "tab" for popup window can be confusing

Problem:    Using "tab" for popup window can be confusing.
Solution:   Use "tabpage". (Hirohito Higashi, closes #4532)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index be86fdc..789c859 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -372,7 +372,7 @@
   call assert_equal(0, bufexists(bufnr))
 
   " global popup is visible in any tab
-  let winid = popup_create("text", {'tab': -1})
+  let winid = popup_create("text", {'tabpage': -1})
   call assert_equal(1, popup_getpos(winid).visible)
   tabnew
   call assert_equal(1, popup_getpos(winid).visible)