E2E: add coverage for Flow and Task system

[zaychenko-sergei]

The test(s) should cover:

• [x] Creating root & derived datasets (using predefined files)
• [ ] Defining flow configurations for updates and observing the triggers
• [x] Triggering flows manually
• [ ] Compacting & resetting flows

Currently, our E2E testing is based on low-level exchange of messages via GraphQL API. Try prototyping higher level test steps that setup the fixture, apply actions and observe results from user point of view.

[s373r]

Within https://github.com/kamu-data/kamu-cli/pull/863, several E2E tests have been added that cover part of the expectations of this ticket:

• Creating root & derived datasets (using predefined files)
• Triggering flows manually

[s373r]

Task accomplished