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

PR-URL: https://github.com/nodejs/node/pull/12054
Ref: https://github.com/nodejs/node/issues/12053
Ref: https://github.com/nodejs/node/pull/11038


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>