Test cloud storage in CI

At the moment, tests for cloud storage (which is just GCP so far) are skipped in CI, because there are no cloud-storage credentials for those tests to use.

The code permits setting credentials and a bucket name for tests, and we could put those in a GitHub secret. However, I'd rather not put credentials I pay for in a secret on this project -- at least not associated with my personal Google account.

Maybe we can find a way to set up a GBF Google account, and use that? I expect this would be within the free tier, especially if we only run these tests in one action (so not once for each Fedora, Ubuntu, etc.). @tbabej is there any precedent for that?