benchmark: terminate child process on Windows

test-benchmark-child-process failures reveal that
child-process-exec-stdout benchmark sometimes leaves around a stray
yes.exe process. Add code to terminate the process.

PR-URL: https://github.com/nodejs/node/pull/12658
Ref: https://github.com/nodejs/node/issues/12560


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>
Reviewed-By: James M Snell <jasnell@gmail.com>