test: fix flaky child-process-exec-kill-throws

Kill the child process with `SIGKILL` to make sure the child process
does not remain alive.

Fixes: https://github.com/nodejs/node/issues/20139

PR-URL: https://github.com/nodejs/node/pull/20213


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: James M Snell <jasnell@gmail.com>