test: fix unreliable async-hooks/test-signalwrap

Use an interval to keep the event loop open so the test does not exit
before receiving all signals fom asynchronous `exec()` calls.

PR-URL: https://github.com/nodejs/node/pull/17827
Fixes: https://github.com/nodejs/node/issues/14070


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>