Use labels for filtering

[mfocko]

Currently we're filtering the PRs that we validate by using the title of the PR.

For having a more systematic approach of validation, it would be ideal to support labels which allows:

• easier filtering of the PRs, if we need to retrigger manually to test
• having an automation-friendly list of tests (they could be parsed for move-stable for example or some CLI status checker)
• having tests that intentionally fail (e.g. deprecations, or testing specific features tied to the failing result of the external CI (such as OSH))