test: fix test-tls-zero-clear-in flakiness

It can happen that the server-side socket is destroyed before the
client-side socket has gracefully closed, thus causing a 'ECONNRESET'
error in this socket. To solve this, also close gracefully in the server
side too.

PR-URL: #4888
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

test/parallel/test-tls-zero-clear-in.js

@@ -22,7 +22,7 @@ var server = tls.createServer({
 }, function(c) {
   // Nop
   setTimeout(function() {
-    c.destroy();
+    c.end();
     server.close();
   }, 20);
 }).listen(common.PORT, function() {