patch 8.1.1521: when a popup window is closed the buffer remains

Problem:    When a popup window is closed the buffer remains.
Solution:   Wipe out the buffer.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 06a33d4..d100dc9 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -272,12 +272,17 @@
 func Test_popup_in_tab()
   " default popup is local to tab, not visible when in other tab
   let winid = popup_create("text", {})
+  let bufnr = winbufnr(winid)
   call assert_equal(1, popup_getpos(winid).visible)
   tabnew
   call assert_equal(0, popup_getpos(winid).visible)
   quit
   call assert_equal(1, popup_getpos(winid).visible)
+
+  call assert_equal(1, bufexists(bufnr))
   call popup_clear()
+  " buffer is gone now
+  call assert_equal(0, bufexists(bufnr))
 
   " global popup is visible in any tab
   let winid = popup_create("text", {'tab': -1})