Commits missing Signed-off-by line – needed for downstream projects with DCO requirements

Hello,

We are in the process of importing gtest-parallel into [Valkey](https://github.com/valkey-io/valkey) as a git subtree. However, Valkey requires all third-party code to comply with the Developer Certificate of Origin (DCO), which means every commit must include a `Signed-off-by` line.

We noticed that many commits in this repository are missing DCO sign-offs, which prevents us from consuming the project without rewriting history.

Would it be possible to update the commit history to include proper Signed-off-by lines? This would allow projects with DCO enforcement to legally and cleanly vendor gtest-parallel.

Thank you for your consideration.

[Related PR](https://github.com/valkey-io/valkey/pull/2956)

— Harry Lin