test(test): limit file permissions

[Fdawgs]

Closes https://github.com/fastify/point-of-view/security/code-scanning/43 by setting file permissions to 0600 (read/write for owner only).

Checklist

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

[Fdawgs]

Test failure unrelated, art-template@4.3.1 was released 6 hours ago and is missing files.