tests: storage: refactor + bump offset.
Merge pull request #151 from stephenplusplus/regression-fails

tests: storage: refactor + bump offset.
storage: fix credentials issue.

Previously, if a user wanted to generate a signed URL, there was
no guarantee a credentials object would have already been
generated. By itself, generating a signed URL doesn't require a
network connection, thus the uncertainty that we would have
already fetched a token. This change requires a valid credentials
object to exist before returning the `credentials` object.