Cover cases where different secrets file are in use

What does this MR do and why?

Cover authentication API failures because incorrect secrets are configured. Users can either have a different secrets file configured, or in some rare cases, none at all, in which case GitLab will default to .gitlab_shell_secret, as seen in L#226 and L#86

How to set up and validate locally

1. Remove the corresponding [gitlab] entry from the Gitaly server's config.toml.
2. Try to push a commit to a repository
3. Gitaly will fail to authenticate with the Rails application and raise a 401. No authentication_token is included in the API request to the /api/v4/internal/allowed endpoint.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.