test: permit test-signalwrap to work without test runner

test/async-hooks/test-signalwrap.js passes with the test.py test runner
but fails if run directly with the `node` executable. Modify the test so
it passes in both cases.

PR-URL: https://github.com/nodejs/node/pull/28306


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>