Reorganize repo layout for test suite images

As a first step towards #8 (closed), reorganize all test suite images into testsuite directory.

This merge request includes following changes:

• Create a copy of buildstream-fedora in testsuite, but without installing BuildStream
• Move buildstream-debian image in testsuite namespace
• Add a helper script to handle build/test/push of images to keep .gitlab-ci.yml short and sweet
• Standardize names of testsuite images - testsuite-foo -> testsuite/foo

Related tasks:

• Create the new images on Docker Hub
• Submit a MR on buildstream/buildstream to adopt these changes