`stretch/sid` in `/etc/os-release` means either testing, or unstable, or a mixture of both, during the Stretch development cycle.
`/etc/os-release` is provided by `base-files`; we don't attempt to keep a testing-specific version, different from an unstable-specific version, so the version in testing and unstable just has the current testing codename and "sid". When testing is frozen before the release, `base-files` is updated with the release name.