Make use of a `gitlab-qa-sandbox` group if available

This will use a top-level group if it's available, or create one if it's not, then create our qa_test_* groups as subgroups of the sandbox group.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/37996

This does not yet fully address @grzesiek's note:

Note that we might want to pass top-level group name using an environment variable, and create one if environment variable is not set to make it work with any instance.