Credit to https://gitlab.com/ejiek for spotting this one.

Adds tests to ensure that JSON deserialisation from program arguments
takes place. This was a testing blind spot, which led to a brief
regression:

https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/280

The reporter struct can be used for passing around and reporting to
the io.Writer of choice.

This allows gitlab-shell to be called with an argument of the format
`key-123` or `username-name`.

When called in this way, `gitlab-shell` will call the GitLab internal
API. If the API responds with user information, it will print a
welcome message including the username.

If the API responds with a successful but empty response, gitlab-shell
will print a welcome message for an anonymous user.

If the API response includes an error message in JSON, this message
will be printed to stderr.

If the API call fails, an error message including the status code will
be printed to stderr.

This adds parsing if the `secret_file` from YAML. And reads the
contents of the configured file.

If no file is configured, we fall back to the `.gitlab_shell_secret`
in the root dir of the configuration.

If the configured path does not start with a `/` we start looking for
the file relative to the root dir of the configuration.