Revamp End-to-End testing

### Main idea

Since Backend.AI project is getting complicated even more these days, it is almost impossible for us to test core functionalities only with our hands. Therefore I think this is the right time to revamp our old fashioned integration test package and introduce a daily E2E testing CI pipeline.

- [x] Update (or rewrite) `ai.backend.test.cli_integration` package so that it can be ran and report errors
- [ ] Add new test cases for other uncovered CLI features
- [ ] Create a Github action pipeline to execute the integration on a daily basis
### Alternative ideas

_No response_

### Anything else?

_No response_

JIRA Issue: BA-175