test: enable mustCall() during child exit

PR-URL: https://github.com/nodejs/node/pull/16915


Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>