patch 8.1.1535: popup select test fails on Mac

Problem:    Popup select test fails on Mac.
Solution:   Skip test if clipboard feature not available.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 6e11bba..79207c8 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -323,6 +323,9 @@
   if !CanRunVimInTerminal()
     throw 'Skipped: cannot make screendumps'
   endif
+  if !has('clipboard')
+    throw 'Skipped: clipboard feature missing'
+  endif
   " create a popup with some text to be selected
   let lines =<< trim END
     call setline(1, range(1, 20))