patch 8.0.1604: paste test may fail if $DISPLAY is not set

Problem:    Paste test may fail if $DISPLAY is not set.
Solution:   Add WorkingClipboard() and use it in the paste test.
diff --git a/src/testdir/test_paste.vim b/src/testdir/test_paste.vim
index 6450989..65f300c 100644
--- a/src/testdir/test_paste.vim
+++ b/src/testdir/test_paste.vim
@@ -6,6 +6,8 @@
 endif
 set term=xterm
 
+source shared.vim
+
 func Test_paste_normal_mode()
   new
   " In first column text is inserted
@@ -67,7 +69,7 @@
 endfunc
 
 func Test_paste_clipboard()
-  if !has('clipboard')
+  if !WorkingClipboard()
     return
   endif
   let @+ = "nasty\<Esc>:!ls\<CR>command"