chore: update tap@18

[climba03003]

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

[jsumners]

Unless things have changed, it's a little bit more involved https://github.com/fastify/fastify/pull/4978/files

[climba03003]

Unless things have changed

Tested locally, it only requires minimal changes.
Is it hard requirement to remove c8?

[jsumners]

tap@18 is using c8 itself.

[climba03003]

After testing on removing the external c8 and using the internal one.
It forced to includes the files that should be ignored in the coverage report.
For example, error-serializer.js.

[gurgunday]

Are macos tests flaky?

[gurgunday]

Seems like only macOS fails with a heap limit error

[gurgunday]

Do you know why macOS unit test CI could be failing? @climba03003