test: kill child in tls-server-verify for speed up

For better performance of the test, the parent kills child processes
so as not to wait them to be ended.

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>