test: mark WASI's test-return-on-exit as flaky

Refs: https://github.com/nodejs/node/pull/36139

PR-URL: https://github.com/nodejs/node/pull/36139


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>