test: fix flaky test-child-process-exec-timeout

At least starting with Darwin Kernel Version 16.4.0, sending a SIGTERM
to a process that is still starting up kills it with SIGKILL instead of
SIGTERM.

PR-URL: https://github.com/nodejs/node/pull/12159
Refs: https://github.com/libuv/libuv/issues/1226


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>