Resolve "spec/lib/gitlab/backend/shell_spec.rb causes other specs to fail"

What does this MR do?

It ensures we rely on Gitlab::Shell's public API and not on its internal. Since Gitlab::Shell is caching the value of its token, the only way to get the correct token is to call Gitlab::Shell.secret_token, not to read the Gitlab.config.gitlab_shell.secret_file file!

Fixes #24599 (closed)