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.

(cherry picked from commit 833b23636045f7afc929196139021630a390391a)

Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/joyent/node/pull/25368