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.

src/testdir/test_popupwin.vim

@@ -2194,6 +2194,8 @@ func Test_previewpopup()
 endfunc
 
 func Test_popupmenu_info()
+  CheckScreendump
+
   let lines =<< trim END
       set completeopt+=preview,popup
       set completefunc=CompleteFuncDict

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1881,
 /**/
     1880,
 /**/