patch 8.2.1237: changing 'completepopup' after opening popup has no effect

Problem:    Changing 'completepopup' after opening a popup has no effect. (Jay
            Sitter)
Solution:   Close the popup when the options are changed. (closes #6471)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 8df01e1..054d479 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -3109,6 +3109,12 @@
   call term_sendkeys(buf, "otest text test text\<C-X>\<C-U>")
   call VerifyScreenDump(buf, 'Test_popupwin_infopopup_7', {})
 
+  " Test that when the option is changed the popup changes.
+  call term_sendkeys(buf, "\<Esc>")
+  call term_sendkeys(buf, ":set completepopup=border:off\<CR>")
+  call term_sendkeys(buf, "a\<C-X>\<C-U>")
+  call VerifyScreenDump(buf, 'Test_popupwin_infopopup_8', {})
+
   call term_sendkeys(buf, "\<Esc>")
   call StopVimInTerminal(buf)
   call delete('XtestInfoPopup')