patch 8.1.1574: tabpage option not yet implemented for popup window

Problem:    Tabpage option not yet implemented for popup window.
Solution:   Implement tabpage option, also for popup_getoptions().
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 7d723a0..451f0b6 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -398,8 +398,10 @@
   let winid = popup_create("text", {})
   let bufnr = winbufnr(winid)
   call assert_equal(1, popup_getpos(winid).visible)
+  call assert_equal(0, popup_getoptions(winid).tabpage)
   tabnew
   call assert_equal(0, popup_getpos(winid).visible)
+  call assert_equal(1, popup_getoptions(winid).tabpage)
   quit
   call assert_equal(1, popup_getpos(winid).visible)
 
@@ -411,11 +413,23 @@
   " global popup is visible in any tab
   let winid = popup_create("text", {'tabpage': -1})
   call assert_equal(1, popup_getpos(winid).visible)
+  call assert_equal(-1, popup_getoptions(winid).tabpage)
   tabnew
   call assert_equal(1, popup_getpos(winid).visible)
+  call assert_equal(-1, popup_getoptions(winid).tabpage)
   quit
   call assert_equal(1, popup_getpos(winid).visible)
   call popup_clear()
+
+  " create popup in other tab
+  tabnew
+  let winid = popup_create("text", {'tabpage': 1})
+  call assert_equal(0, popup_getpos(winid).visible)
+  call assert_equal(1, popup_getoptions(winid).tabpage)
+  quit
+  call assert_equal(1, popup_getpos(winid).visible)
+  call assert_equal(0, popup_getoptions(winid).tabpage)
+  call popup_clear()
 endfunc
 
 func Test_popup_valid_arguments()