Dedicated test project for openQA-devel installability verification (openQA-devel-test).

Background & Motivation:
openQA-devel pulls in all build, check, and test requirements for openQA development, including chromedriver and chromium for Selenium tests. Upstream chromium rebuilds take significant time and frequently leave openQA-devel-test in a 'blocked' state.

When openQA-devel-test was built inside devel:openQA via _multibuild, its blocked state prevented repository publication of devel:openQA (state="blocked"), causing:
1. openqa-auto-update.service on production instances (e.g. o3 / openqa.opensuse.org) to fail when openqa-check-devel-repo checks repository status.
2. Fleet rollouts and osd-deployment pipelines to be blocked.

By isolating openQA-devel-test into this sub-project (which builds against binaries published by devel:openQA), installability of development dependencies is continuously tested without blocking production package publication and deployments.

See: https://progress.opensuse.org/issues/208263