Revamp End-to-End testing

[kyujin-cho]

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.

• 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

[rapsealk]

@mirageoasis You may start a new PR from #1659 if you don't mind.