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.

PR-URL: https://github.com/nodejs/node/pull/3966


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>