Add tests with absolute minimum code change

[tyrannosaurus-becks]

Hello! Thanks for creating this repository and making it public!

In light of https://github.com/actions/cache/issues/810 being open long-term, we'd like to move over to using this action. I am a Go developer and would like to harden this repo's code first. I thought I'd start by adding test coverage for the code's existing state, and after that exists, I might follow up with a second PR further hardening the code.

If that seems cool with you, it would be awesome if you'd consider this PR. Please let me know if there's anything you'd like me to change to make it better, I'd be happy to do that.

Thanks in advance! 😄 ❤️

[tyrannosaurus-becks]

@gmsantos @amendoa @brunopadz if any of you are available to check this out it would be much appreciated.

[gmsantos]

@tyrannosaurus-becks sorry I'm not a maintainer. At the moment, I think @amendoa is the only maintainer.

[brunopadz]

Same here @tyrannosaurus-becks, not a maintainer anymore.