patch 8.1.1908: every popup window consumes a buffer number

Problem:    Every popup window consumes a buffer number.
Solution:   Recycle buffers only used for popup windows.  Do not list popup
            window buffers.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 358da6d..f742035 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2331,4 +2331,13 @@
   call delete('XtestInfoPopupNb')
 endfunc
 
+func Test_popupwin_recycle_bnr()
+  let winid = popup_notification('nothing wrong', {})
+  let bufnr = winbufnr(winid)
+  call popup_clear()
+  let winid = popup_notification('nothing wrong', {})
+  call assert_equal(bufnr, winbufnr(winid))
+  call popup_clear()
+endfunc
+
 " vim: shiftwidth=2 sts=2