Bump undici from 5.28.5 to 5.29.0 in /.github/actions/tests/split_tests

Bumps [undici](https://github.com/nodejs/undici) from 5.28.5 to 5.29.0.
- [Release notes](https://github.com/nodejs/undici/releases)
- [Commits](nodejs/undici@v5.28.5...v5.29.0)

---
updated-dependencies:
- dependency-name: undici
  dependency-version: 5.29.0
  dependency-type: indirect
...

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Author: dependabot[bot]