Bump orchestra/testbench from 8.15.0 to 8.21.1

Bumps [orchestra/testbench](https://github.com/orchestral/testbench) from 8.15.0 to 8.21.1.
- [Release notes](https://github.com/orchestral/testbench/releases)
- [Changelog](https://github.com/orchestral/testbench/blob/develop/CHANGELOG-8.x.md)
- [Commits](orchestral/testbench@v8.15.0...v8.21.1)

---
updated-dependencies:
- dependency-name: orchestra/testbench
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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