In order to empower reuse of the tests from engineering-tests, it's easier to have them in one plainbox-provider. They are all _provided_ by the same repository, so it IMHO makes more sense to have them grouped like that. Other (minor) benefit is having a lot less parts in snapcraft.yaml.
From a user's perspective nothing should change. I modified all the 'apps' to reflect the change in namespaces.
Potential follow up:
1) I left the alsa-utils reference untouched. The tests from A-U repo could live here, referencing, and requiring the A-U snap. This would deduplicate quite a bit.
2) All the 'apps' use plainbox run with a regex from commandline. I think having clearly defined "Test Plans" could be easier to use, maintain and - most importantly - reused.
3) Having a mix of dedicated 'launchers' (in ./bin) and direct calls could be improved, especially as a part of 2)
4) Theoretically plainbox should not be called. checkbox-cli should be used instead. This doesn't have any major importance here, as the benefits wouldn't be used (auto re-running of automated tests, bootstrapping test plans, ...)