patch 8.0.0507: client-server tests fail when $DISPLAY is not set

Problem:    Client-server tests fail when $DISPLAY is not set.
Solution:   Check for E240 before running the test.
diff --git a/src/testdir/test_clientserver.vim b/src/testdir/test_clientserver.vim
index c98fc02..58ced56 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -11,6 +11,17 @@
   if cmd == ''
     return
   endif
+  if has('unix')
+    try
+      call remote_send('xxx', '')
+    catch
+      if v:exception =~ 'E240:'
+	" No connection to the X server, give up.
+	return
+      endif
+      " ignore other errors
+    endtry
+  endif
 
   let name = 'XVIMTEST'
   let cmd .= ' --servername ' . name