Tests doc
Add sid container images for tests
The default image for tests were based on Debian stable. It's good to
test patches that apply for production service, but not good for test
new upload for Debian.
For test new OBS packages before upload into Debian. We need to have
sid images. So that here we add these sid images.
The obs-tests/sid/README.md file contains the documents for how to
build and use the sid image with step by step command and scripts.