Accept `github` option for `--ci` when scaffolding plugin and theme tests

## Feature Request

- [x] Yes, I reviewed the [contribution guidelines](https://make.wordpress.org/cli/handbook/contributing/).

**Describe your use case and the problem you are facing**

Currently Github Action is not supported as valid CI in scaffold plugin-tests and scaffold theme-tests.

**Describe the solution you'd like**

It would be great if could add support for GitHub actions.
