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/testdir/shared.vim

@@ -260,6 +260,16 @@ func CanRunGui()
   return has('gui') && ($DISPLAY != "" || has('gui_running'))
 endfunc
 
+func WorkingClipboard()
+  if !has('clipboard')
+    return 0
+  endif
+  if has('x11')
+    return $DISPLAY != ""
+  endif
+  return 1
+endfunc
+
 " Get line "lnum" as displayed on the screen.
 " Trailing white space is trimmed.
 func! Screenline(lnum)

src/testdir/test_paste.vim

@@ -6,6 +6,8 @@ if has('gui_running')
 endif
 set term=xterm
 
+source shared.vim
+
 func Test_paste_normal_mode()
   new
   " In first column text is inserted
@@ -67,7 +69,7 @@ func Test_paste_insert_mode()
 endfunc
 
 func Test_paste_clipboard()
-  if !has('clipboard')
+  if !WorkingClipboard()
     return
   endif
   let @+ = "nasty\<Esc>:!ls\<CR>command"

src/version.c

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