Artefacts conflating snap revisions from multiple channels based on version strings

Steps:
- have snap r1 `v1.0` on `beta`
- release snap r2 `v1.0` on `edge`

Expected:
- r1 visible on `beta`

Current:
- r2 visible on `beta`

Snap version strings are only meant to be human readable (contrary to debs). Test observer should not consider revisions not released to certain channels as part of the same artefact solely based on their version string. The channel on which the revision was published needs to be considered as well.