Bump slab from 0.4.10 to 0.4.11 in /tests/deps

Bumps [slab](https://github.com/tokio-rs/slab) from 0.4.10 to 0.4.11.
- [Release notes](https://github.com/tokio-rs/slab/releases)
- [Changelog](https://github.com/tokio-rs/slab/blob/master/CHANGELOG.md)
- [Commits](tokio-rs/slab@v0.4.10...v0.4.11)

---
updated-dependencies:
- dependency-name: slab
  dependency-version: 0.4.11
  dependency-type: indirect
...

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Author: dependabot[bot]