chore: remove .npmignore to publish test/ directory

[Tony133]

Proposal:

Remove .npmignore so the test/ directory is included in the published npm package.

Why
.npmignore listed test/, so the tests were not shipped with the package. Deleting the file makes npm fall back to .gitignore, which does not exclude test/.

Verification

• npm pack --dry-run now lists the test/ directory in the tarball
• No unwanted files (IDE config, coverage, etc.) are included

Notes:

• See the internal discussion: https://github.com/orgs/fastify/discussions/103