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.

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1604,
 /**/
     1603,
 /**/