tests: Test the ci-package-builder pipeline

Set up a test repository and run the ci-package-builder pipeline on it by committing new changes, creating merge requests and monitoring the resulting pipelines.

The test currently checks:

• submitting non-release changes and landing them
• submitting release commits and landing them
• blocking commits to frozen stable branches

This pipeline needs some extra setup:

• a tests/dash> repository, forked from pkg/target/dash>, where we force-push changes, create merge requests and monitor pipelines
• the OBS_PEFIX CI variable to be set on tests/dash> to point to throwaway OBS branches to test the upload and monitorig jobs, for instance home:apertis-gitlab:branches:test:
• the GITLAB_CI_USER, GITLAB_CI_PASSWORD, OSC_USERNAME and OSC_PASSWORD CI variables to be set on tests/dash>, matching what it is used on the pkg/ projects
• the GITLAB_AUTH_TOKEN CI variable to be set on this repository to access the GitLab APIs used to issue MRs and monitor pipelines, and to push changes via git to the tests/dash> repository

Another extra bit of configuration was needed to be able to run the pipeline from non-default branches:

1. open https://gitlab.apertis.org/admin/application_settings/network#js-outbound-settings
2. enable Allow requests to the local network from web hooks and services
3. add gitlab.apertis.org to the allowlist