patch 8.1.1536: popup select test still fails on Mac

Problem:    Popup select test still fails on Mac.
Solution:   Set 'clipboard' to "autoselect"
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 79207c8..be86fdc 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -328,6 +328,7 @@
   endif
   " create a popup with some text to be selected
   let lines =<< trim END
+    set clipboard=autoselect
     call setline(1, range(1, 20))
     let winid = popup_create(['the word', 'some more', 'several words here'], {
 	  \ 'drag': 1,