Add tests for GCS authentication

[kevinzwang]

In #2393 we added three additional code paths for authenticating to Google Cloud: credentials file path, credentials string, and oauth2 token. It would be good to write integration tests for them.

We already have Google Cloud auth set up in our github actions, we will just need to obtain the file path from the GOOGLE_APPLICATION_CREDENTIALS environment variable and manually pass those in instead of letting the google cloud crate infer it.