patch 8.2.3094: Test_popup_atcursor_pos() fails without the conceal feature

Problem:    Test_popup_atcursor_pos() fails without the conceal feature.
Solution:   Add a check for the conceal feature. (Dominique Pellé,
            closes #8505)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index cdfa3a9..8e02ef1 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1437,6 +1437,7 @@
 
 func Test_popup_atcursor_pos()
   CheckScreendump
+  CheckFeature conceal
 
   let lines =<< trim END
 	call setline(1, repeat([repeat('-', 60)], 15))