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: https://github.com/nodejs/node/pull/4888


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