chore(deps-dev): bump undici from 6.20.1 to 6.21.1 in /tests

Bumps [undici](https://github.com/nodejs/undici) from 6.20.1 to 6.21.1.
- [Release notes](https://github.com/nodejs/undici/releases)
- [Commits](nodejs/undici@v6.20.1...v6.21.1)

---
updated-dependencies:
- dependency-name: undici
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]