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.

Fixes: https://github.com/nodejs/node/issues/14070

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test async_hooks