test: fix tls-multi-key race condition

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

test/parallel/test-tls-multi-key.js

@@ -35,8 +35,8 @@ var server = tls.createServer(options, function(conn) {
       rejectUnauthorized: false
     }, function() {
       ciphers.push(rsa.getCipher());
-      ecdsa.destroy();
-      rsa.destroy();
+      ecdsa.end();
+      rsa.end();
       server.close();
     });
   });