Yeah, you're right, there is a tool called act[1] that lets users test the GitHub Actions CI runs toward their local machine. The implementation looks very similar to GitHub Actions official runner, e.g use the identical provisioning scripts[2]. It is a very nice tool but serves a different purpose than us.

Regarding general testability with GitHub Actions, I'd recommend checking out the tmate action[3], which lets you debug your CI run with SSH.

[1] https://github.com/nektos/act

[2] https://github.com/actions/virtual-environments

[3] https://github.com/mxschmitt/action-tmate