test: make verify_clean_boot respect return code by series

[holmanb]

Proposed Commit Message

test: make verify_clean_boot respect return code by series

Also update stdout / stderr output format.

Additional Context

Failing in https://github.com/canonical/cloud-init/actions/runs/10819125394/job/30016291006?pr=5691

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[TheRealFalcon]

LGTM!

[holmanb]

@TheRealFalcon @a-dubs I just re-requested review, because I had to fix the failing tests. This fix is kinda funky, but basically our integration tests are unpatched, so despite running on focal, they behave like they are running on Noble, so CURRENT_RELEASE doesn't tell us everything that we need to know. This should only really be a problem on main, which is why I think it's safe to do it this way.