refactor(test/index): use Object.hasOwn()

[Fdawgs]

Replaces Object.prototype.hasOwnProperty.call() with the shorter alias Object.hasOwn(), which can now be used as support for older Node versions was dropped with Fastify v5.

See related MDN page.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Fdawgs]

Thanks for reviewing @gurgunday, I think you were the one who recommended this in an issue/PR a while back but I couldn't find it to link to. 😄