test: running tls-server-verify clients in parallel

OpenSSL s_client introduces some delay on Windows. With all clients
running sequentially, this delay is big enough to break CI. This fix runs
the clients in parallel (unless the test includes renegotiation),
reducing the total run time.

Fixes: https://github.com/nodejs/io.js/issues/1461
PR-URL: https://github.com/nodejs/io.js/pull/1836


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>