chore(pipeline): split Build and Test stage

[lemeurherve]

This PR splits the existing stage into "Build" and "Test".

Having separate stages allows a better understanding of the potential gains on each of those part when improving the building process.

Notes:

• Generated docker compose files are archived even if the tests fail
• As the publish target includes building the images, no need to call it first when only that one is running on trusted.ci.jenkins.io
• As there is no "test" phase for multi-arch build, no need to change it

Similar to:

• chore(pipeline): split Build and Test stage docker-agents#1071

Testing done

• CI

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

PS: easier to review without whitespaces, see https://github.com/jenkinsci/docker-agent/pull/1071/files?diff=unified&w=1