patch 8.1.1881: popup window test fails in some configurations

Problem:    Popup window test fails in some configurations.
Solution:   Check that screendumps can be made.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 5de7255..07ed81b 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2194,6 +2194,8 @@
 endfunc
 
 func Test_popupmenu_info()
+  CheckScreendump
+
   let lines =<< trim END
       set completeopt+=preview,popup
       set completefunc=CompleteFuncDict