Bump orchestra/testbench from 6.13.0 to 6.15.0

Bumps [orchestra/testbench](https://github.com/orchestral/testbench) from 6.13.0 to 6.15.0.
- [Release notes](https://github.com/orchestral/testbench/releases)
- [Changelog](https://github.com/orchestral/testbench/blob/6.x/CHANGELOG-6.x.md)
- [Commits](orchestral/testbench@v6.13.0...v6.15.0)

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

Author: dependabot[bot]