test: fix tls-inception flakiness

When sending a very large buffer (400000 bytes) the test fails due to
the client socket from the `a` server erroring with `ECONNRESET`.
There's a race condition between the closing of this socket and the `ssl`
socket closing on the other side of the connection. To improve things,
destroy the socket as soon as possible: in the `end` event of the `dest`
socket.

PR-URL: https://github.com/nodejs/node/pull/4195


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Fedor Indutny <fedor@indutny.com>