test: fix flaky async-hooks/test-graph.signal

Make sure event loop remains open long enough for signal to be received.

PR-URL: https://github.com/nodejs/node/pull/17509
Fixes: https://github.com/nodejs/node/issues/14568


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>